I won a game where I cast at least 5 vanilla creatures and it did not trigger Death by Vanilla.
Here are some extra details: Was playing on Mobile.
As you can see there are no creatures in here with abilities. I do have ways to give creatures abilities AFTER I cast them, but the achievement doesn't say the creatures have to never gain gain abilities or not enter with abilities. It says they only creature spells you cast must have no abilities and you must cast at least 5 creature spells. I did. My plan was cast creatures then give them abilities afterwards with anthems and perpetual abilities. But I never recast them after giving them perpetual abilities.
Here is my deck list. (So You can check for unexpected interactions that aren't working as intended.)
Deck
4 Utopia Sprawl (WOT) 63
3 Yargle and Multani (MOM) 256
2 Yargle, Glutton of Urborg (MUL) 84
4 Garruk's Uprising (FDN) 220
1 The Great Henge (ELD) 161
2 Isamaru, Hound of Konda (JMP) 113
4 Indatha Triome (IKO) 248
2 Underground Mortuary (MKM) 271
3 Gigantosaurus (ANB) 96
4 Deathcap Glade (VOW) 261
3 Razorverge Thicket (ONE) 257
4 Hushwood Verge (DSK) 261
3 Wastewood Verge (DFT) 268
2 Kalakscion, Hunger Tyrant (DFT) 93
2 Leatherback Baloth (WWK) 107
2 Brave the Sands (KTK) 5
1 Always Watching (SIR) 10
3 Temple Garden (GRN) 258
1 Forest (FIN) 306
2 Sundial, Dawn Tyrant (DFT) 31
1 Whip of Erebos (PIO) 119
1 Sorin, Vengeful Bloodlord (WAR) 217
1 The Skullspore Nexus (LCI) 212
4 Monoist Gravliner (Y25) 11
1 Susur Secundi, Void Altar (EOE) 259
1 The Seriema (EOE) 35
1 Squadron Carrier (Y25) 2

Sideboard
1 Quakestrider Ceratops (FDN) 110
1 Diplomatic Relations (EOE) 177
1 Animist's Might (MAT) 20