(As this Saga enters and after your draw step, add a lore counter. Sacrifice after III.)
I — Gain control of target non-Mutant creature for as long as you control Vault 87.
II — Put a +1/+1 counter on target creature you control. It becomes a Mutant in addition to its other types.
III — Draw cards equal to the greatest power among Mutants you control.
If Vault 87 leaves the battlefield before its first chapter ability resolves, you won't gain control of the target creature at all.
08-03-2024
wotc
The target of Vault 87's first chapter ability only needs to be a non-Mutant when that ability is put onto the stack and when that ability resolves. The control effect won't end if it later becomes a Mutant (perhaps because of the effect of the second chapter ability).
08-03-2024
wotc
Use the greatest power among Mutants you control as Vault 87's last ability resolves to determine how many cards to draw.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.