Does Olivia Voldaren's ability to steal a creature need to be activated after it's already a Vampire or at the same time?

Olivia Voldaren has two relevant activated abilities. The first deals 1 damage to a creature and turns it into a Vampire in addition to its other types. The second ability — costing {3}{B}{B} — steals a target Vampire creature permanently. The key requirement is that the target must be a Vampire when the ability resolves, not merely when it is activated.

Under CR 608.2b, if a targeted permanent is no longer a legal target when an ability tries to resolve, the ability is countered and does nothing. This means the creature must have the Vampire subtype at resolution time. If you activate the steal ability targeting a creature that is not yet a Vampire, the ability will be countered on resolution because the target is illegal.

The correct sequence is: first activate Olivia's first ability to deal 1 damage and make the creature a Vampire, let that resolve, and then activate the steal ability targeting that now-Vampire creature. You cannot shortcut this into a single step — both abilities are separate stack objects that resolve independently (CR 112.3a).

Note also that per CR 114.3, the legality of a target is checked both at activation and at resolution. So the targeted creature must be a Vampire both when you activate the steal ability and when it resolves.

Example: Your opponent controls a 3/3 Human. You activate Olivia's first ability targeting it; it resolves, becoming a Vampire. You then activate Olivia's second ability targeting that 3/3 Vampire. When the steal ability resolves, it finds a legal Vampire target and you gain control of it permanently.