Strefan, Maurer Progenitor has the following ability:
Whenever Strefan attacks, you may sacrifice two Blood tokens. If you do, you may put a Vampire card from your hand onto the battlefield tapped and attacking. It gains indestructible until end of turn.
Does the Vampire put onto the battlefield this way has to attack the same player or planeswalker?
Geist of Saint Traft has a specific rule about it:
You choose which player or planeswalker the Angel token is attacking. It doesn’t have to be attacking the same player or planeswalker that Geist of Saint Traft is attacking.