I would also tend to agree that when Barnabas "vanished" into PT, his hold over Sabrina vanished. Of course, she could've always left to go look for Barnabas, and Chris and Amy went along for the ride for whatever reason.
The power a vampire has over their victim, in the DS world anyway, I think has a lot to do with what the vampires intentions are for the victim. If the vamp bites the vic because they need a servant, than the vic will be practically a slave to the vamp's will--just like Willie was at first. If the vamp just needs a quick bite to eat, then the vic will have some loyalty to the vamp, but will not throw themselves in front of the train to get to the vamp, unless, of course, the vamp is summoning their victim. And if the vamp bites the vic in order to keep them quiet and place them under control to prevent them from spoiling some secret, then its a mix of the two.