Vault 87: Forced Evolution
(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 this Saga. 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.
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Official rulings (3) · Scryfall
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).
If Vault 87 leaves the battlefield before its first chapter ability resolves, you won't gain control of the target creature at all.
Use the greatest power among Mutants you control as Vault 87's last ability resolves to determine how many cards to draw.