Could be nice complements

Two-Headed Cerberus --> Raging Redcap -- Less mana intensive, now almost playable!
Hellkite Punisher --> Terror of Mount Velus -- Ooooo, fun! Velus comes down from the Evergreen cube. Big add to Scourge of the Undercity, Cavalry Drillmaster, Rage-Scarred Berserker, and Dinotomaton line.
Iroas's Champion --> Swiftblade Vindicator -- Oh this was tough. I don't like cutting Champion, especially given red's lack of low mana value oomph. Makes me question keeping all two color pairs capped at 5 cards even if multiple cards are hybrid, as is RW. Hmmm. Anyway, Vindicator is sick, just gotta pump it somehow!
Gorgon Flail --> Basilisk Collar -- Yesssssss!!!!! Flail is meat and potatoes, but Collar is premium, love it, even better for pingers.
Suntail Hawk --> Healer's Hawk -- Healer's Hawk comes down from Evergreen Cube, Suntail Hawk gets bumped to Vanilla, long live reminder text!
Dawnfeather Eagle --> Syr Alin, the Lion's Claw -- I love Eagle, fun, and teachy. Syr Alin gives white a five mana creature with first strike, different from the flyers and the disenchant doggy. I feel mixed on adding attack triggers, but red already has them so what the heck.
Explosive Impact --> Goblin Oriflamme -- I resisted Oriflamme for a long time because of the attacking creatures clause, but I already have the same with Gruul War Chant, so why not? Let's encourage players to attack. Impact was unspectacular and not unique.
Goblin Arsonist --> Bloodlust Inciter -- Fine, I'll do it. Death triggers are on the higher end of board complexity for the cube, okay to cut. I'd rather have a 1/1 creature with first stike, menace, or even trample, oh well.
Careless Celebrant --> Axgard Cavalry -- Really wish this was Nest Robber, but we can't have everything we want, this is close enough.
Sabretooth Tiger --> Giant Cindermaw -- Again, I really wish this was a 2/1 first strike creature, but not even Foundations can give us our heart's desires. Cindermaw's lifegain hate feels weird but also grokable to new players and pairs well with Felidar Sovereign. Getting a +2/+2 bump to a 4/3 body feels GREAT! Red deck wins inches closer.
Siegebreaker Giant --> Wrecking Crew -- Coin flip on this one. I like the aggressive body and mana sink for the giant, but Lambholt Harrier has the same ability. Crew is less aggressive and has reach, could be grindy, but I like how clean it is with the two abilities.
Kessig Recluse --> Magnigoth Sentry -- Cutting an ability, you say? Somewhere, Giant Spider is crying. Yes, Recluse has deathtouch, but Sentry attacks. Another toss up.
Lifespring Druid --> Three Tree Rootweaver -- I loathe to switch from 2/1 to 1/3 but it is a mana cheaper and as an added bonus I will take cute critters over sexy elves any day for inclusive purposes.
Tamiyo's Safekeeping --> Ambush Wolf -- I want Safekeeping's life gain to go away, but even still, it's a wall of text. Wolf is weird! Flash is fun, only other green flasher is Ashcoat Bear, and things just haven't been the same since Ambush Viper left town and went to the Evergreen Cube. With all this mill running around, having a single graveyard hate card seems appropriate.
Watchwolf --> Anthem of Champions -- Aggggh, hard. Love Watchwolf. They should give it trample or vigilance. Anthems of Champions is arguably redundant with Collective Blessing, but really they're quite different. We'll see!
Conjurer's Closet --> Quick-Draw Katana -- Closet was on the bubble for complexity. Katana can't match the simplicity of Viridian Claw or No-Dachi, but I understand the desire to limit first strike to an attacking ability to avoid board stalls. I accept these terms.
Arborback Stomper --> Elvish Regrower -- Stomper? Really? Love that beast. We'll miss the life gain in green. Regrower helps ... regrow the 4 MV spot for green, complementing Eternal Witness with a beefier body while providing payoff for the growing mill movement.

Tempting, but I'm worried that new players will get locked out if they don't have removal, unfun.

Dawnstrike Paladin --> Felidar Sovereign -- Well, well, well, an alt-win con in the cube! Sovereign's beefier stats help actually end games instead of just stalling out. Paladin was one of my least favorite cards but felt hard to cut because of its unique ability pairing, but was really quite slow.
Breaching Hippocamp --> Chrome Prowler -- Drop a mana and tap their creature instead of yours means more games end faster. Artifact typing might be slightly confusing but I'm leaning into it.
Disallow --> Opportunity -- I always want to include Disallow since cutting Stifle but it there are few instances to target an activated or triggered ability instead of the spell itself. Anyway, Opportunity helps round out the Opt Quick Study Jace's Ingenuity pipeline.
Malfunction --> Starlight Snare -- Doesn't hit artifacts but is a mana cheaper and doesn't have the double blue of Claustrophobia, I'll take it!
Annul --> Malcator's Watcher -- Finally giving up on having counterspells for the card types, as they consistently rode the pine in the sideboard. Let's add a blue creature with vigilance that will see play instead.
Envelop --> Vexing Gull -- See above re card type counterspells. Vexing Gull is sick!
Wall of Air --> Wall of Lost Thoughts -- The difference between 1 power and 0 power is subtle but important, and when you include taking away flying, Wall of Lost Thoughts is less likely to make the game grindy. More importantly, I'm leaning into mill, and this is an easy add.
Markov Patrician --> Koma's Faithful -- Plus mill!
Child of Night --> Scourge of the Undercity -- See Cavalry Drillmaster, Rage-Scarred Berserker, and Dinotomaton.
Vampire Champion --> Venomous Hierophant - Plus mill!
Shatterskull Recruit --> Resistance Skywarden -- +1/+1, add reach, auto add.
Lightning Elemental --> Voldaren Duelist -- Consolidated stats but I've wanted Duelist for a while, pleased to add it to the cube, makes for a nice on-curve can't block chain with Goblin Shortcutter and Unstoppable Ogre.
Greenwood Sentinel --> Brushstrider -- Oh hell yeah, I've wanted Brushstrider forever! Square stats are grokable and much better than greater toughness than power, but I like the mini glass cannon of a 3/1 body for gameplay purposes.
Confidence from Strength --> Predator's Strike -- Yep! Finally, a green combat trick with trample reminder text! I must be literally the only person on the planet who cares about these things.
Sentinel Spider --> Tajuru Pathwarden -- Pathwarden is another card I've wanted to add for years. Less mana intensive and extra power, nice. Reach for trample I'm indifferent on, except Arborback Stomper becomes cuttable.

Bad card, yet unique! Cut Yargle, send him to Vanilla???

Passwall Adept --> Piranha Fly -- Adept's ability is redundant with Wormhole Serpent. I love Fly! So long, "enters the battlefield." It enters tapped and has flying? Let's race! Fast games = good games.
Essence Extraction --> Winter's Intervention -- Less mana intensive, kills 151 instead of 199 of the cube's 238 creatures.
Skymarch Bloodletter --> Crow of Dark Tidings -- Part of me wants to have a three-tier cycle with Skymarch Bloodletter, Bloodhunter Bat, and Vampire Sovereign, but I'll instead opt for the corvid's diversity, as no other creature mills.
Revoke Existence --> Exorcise -- Power creep and mainboardability, complements Make Your Move.
Disentomb --> Corpse Churn -- The continued complexity creep of the cube. Disentomb is really feeling like a Vanilla Cube card (there's already Raise Dead), Churn brings more mill to the fold.
Vedalken Dismisser --> Bellowing Crier -- Ugh, my heart. I love Dismisser's unique ability, but at 6 it is overcosted for today's magic. Crier will actually hit the board and attack well, perhaps spark a conversation comparing it to Merfolk Looter.
Eyes of the Beholder --> Withering Torment -- Eyes of the Beholder is somewhat unique and is a fun flavor card for D&D folks, but it is overkill (pardon the pun). There are a few other answers for indestructible creatures. Torment introduces the cube to black's foray into enchantment removal as well as losing life on a kill spell. Wild!

With all the self-mill, do I need to add more graveyard synergy? Codex Shredder is intriguing.

Lost Legion --> Ravine Raider -- I have a soft spot for Lost Legion because it's unique as a black creature that scrys, but its two black pips and greater toughness than power gum up the game on multiple axes. Raider's menace is a new keyword ability for a black one drop, and the shade ability isn't too much.

Lots of cards that didn't quite make the cut!
Alania's Pathmaker -- Maybe if it was this turn, but next turn is a memory issue.
Bellowing Crier -- Close! Maybe cut Vedalken Dismisser. The Evergreen Cube cares about card draw, and the Deciduous Cube cares about draw and discard, so I might put it there.
Daggerfang Duo -- Gimme that deathtouch reminder text and it's in.
Mabel's Mettle -- Right wheelhouse but not as good of a fit as Rallying Roar or Join Forces.
Skyskipper Duo -- Ugh, maybe? Lots of words but it would be a unique effect for a blue creature and would pair nicely with the cube's many ETB (E?) effects.
Rabid Gnaw -- Can we get a cheap red fight with reminder text, please?
High Stride -- Strictly better than Vines of the Recluse but no reminder text.
Stickytongue Sentinel -- Hi Invasive Species!
Three Tree Rootweaver -- Lifespring Druid. What slows down the game more, a 3 toughness creature on turn 2 or not casting your ramp until turn 3?
Pond Prophet -- Hmmm. Where to put you?

The Vanilla Cube is inheriting a lot of sorceries from the French Vanilla Cube. Each cube is getting more complexity. I'm trying to show more of what's possible by caring more about mill and spells shenanigans.

Switcheroo --> Shifting Loyalties -- Switcheroo to the Vanilla Cube, Shifting Loyalties returns!
Nausea --> Shrivel --Shrivel and Nausea swapped cubes. I like the edgier printing in the French Vanilla Cube.
Sovereign's Bite --> Unseal the Necropolis -- SB was weak, Unseal leans into mill, complements Another Chance to help make recursion a thing.
Diabolic Tutor --> Summon Undead -- Speaking of recursion! Tutor was meh, I like Summon Undead a lot.
Roast --> Shunt -- I've been steadily reducing the amount of removal in the French Vanilla Cube to allow for more diversity of card effects. This is admittedly a big creep in complexity, but will allow for interesting interactions, as I really want to see Sorin's Vengeance or Wit's End get bounced back in someone's face. Roast will do well in the Vanilla Cube, and I expect Shunt to be a card that more experienced players use to teach the noobs how the stack works.
Void Snare --> Redirect -- Void Snare to Vanilla Cube. Redirect is technically better than Shunt as it can interact with spells that have multiple targets.
Schismotivate --> Swerve -- I love that Swerve is the same effect, same mana value, one blue one red. Fun!
Waterwhirl --> Twincast -- I always confuse the copy effects with the change targets effects, probably because there are a few cards that do both. As I have pushed sorceries in the Vanilla Cube, I've been leaning into the idea that the French Vanilla Cube is where instants are introduced. Copying spells isn't something that I don't do often as a player, but it seems fun! Waterwhirl felt expendable.
Pyroclasm --> Reverberate -- I'm a little upset to see Pyroclasm go, but it was needed in the Vanilla Cube. Reverberate is Twincast's twin!

Captivating Gyre is a stronger card IMO but it went to the Vanilla Cube and Waterwhirl is the right card to replace it.

Lure to Vanilla Cube, Explosive Vegetation rounds out a nice yin & yang for green sorceries and auras that fix/ramp, each having four across mana values 1-4.

Wild Instincts comes out because even though I prefer the square stats, the increased number of experienced players playing the cube meant I needed to lean into power a bit more. Epic Confrontation costs 2 less, and is a neat story piece.