This hypothetical doesn't quite make sense to me, because I see these vows as actually beneficial for practice and realization, since they strike at the root of attachment to the self and its priorities (eg "getting rid of suffering"). It's a kind of a paradox I've also heard from Thai Forest teachers - embracing suffering, one puts an end to it. Letting go of one's concern for liberation in favour of the liberation of everybody else, one brings it closer.
As for delaying nirvana, this interpretation of the vows is just one interpretation, and one that doesn't completely make sense to me (and is also rejected by most Vajrayana people).
Then on the other hand I've heard a teacher share that in the course of meditation the vow came back and barred the entry (or rather the exit), so to speak. So it varies.
[Edit: If you make explicit vows to keep getting reborn in order to be of assistance to other beings, well in order to pull all that off I imagine you'd already have to be very close to Buddhahood, but I wouldn't like to speculate on the actual workings of all that).]
I've done some work in prisons and my wife works with people with disabilities, so this amazing good fortune that we have is something I don't want to take for granted. Striving just for my own sake strikes me as kind of insensitive and even a bit pointless (I am an extrovert). This is the meaning of the Vows for me (No, Tilt, I am not saying Theravada practitioners practice only for themselves (I have no idea of other people's motivation), I am just talking about my practice