Oathsworn Vampire only triggers once.

If I trigger an Oathsworn Vampire by gaining life, and I have 3 creatures in play, I am asked if I want to replace one of my creatures (Yes/No?).

If I say no, then I do not get the chance to put the Oathsworn Vampire in play on a later turn, if I gain enough life.

Possibly the Oathsworn Vampire is being removed from the graveyard? I cannot tell. It should not be, however.