When Attendant of Vraska dies, if you control a Vraska planeswalker, you gain life equal to Attendant of Vraska's power.
If Attendant of Vraska’s power was negative, you don’t lose or gain life.
If you don’t control a Vraska planeswalker as Attendant of Vraska’s triggered ability resolves, possibly because Vraska left the battlefield at the same time as Attendant of Vraska, you won’t gain life.
The amount of life gained is equal to Attendant of Vraska’s power as it last existed on the battlefield.