First off, shame on the ST for trying to get you guys to RP it. If that kind of scene causes the players discomfort, then you drop a curtain on it, full stop.

That said, you can resolve this with a simple question: How human is your vamp?

I'll explain. For an ordinary human, there are several options available in terms of one's psychological reaction to such an event, and none of them do well for party cohesion.

But vampires aren't human. They look at reality, at life (or un-life) through an entirely different lens. For example, vampires know for a fact that there exists a power that allows them to utterly control the mind and will of others. They accept this, for the most part; to many vampires, Dominate is as natural a thing to do as engaging in conversation. It's simply a thing one does, like walking, talking, or drinking blood.

Which raises another issue. It really forces a person to redefine concepts of right and wrong, and personal violations and such, when one must feed on the blood of the living to survive.

I'm not saying your character should be unaffected by what happened; far from it. What I'm saying instead is that there are more options available to you than the standard array, which seems to consist either of hiding from or becoming violent towards the offender in question. Being a vampire, there is a bigger world out there, and you see things more broadly. You have centuries in which to sort out what has happened, rather than mere years.

I suggest vengeance. Methodical, cold, calculating vengeance. Not swift, not sudden, and definitely not foreseeable. Don't set aside your planned character progression; instead, add a dimension to it. You say the character is a nerdy, shy, bookworm-type, the kind with a fascination with and a penchant for knowledge. Well, now it's not just knowledge for its own sake; this character can have a new goal, a darker one, in addition to her prior aims. In the alternative, developing a healthy sense of paranoia isn't a terrible idea for any vamp, particularly one who just experienced something this disturbing. And being a bookish sort, it shouldn't be too hard to research all sorts of exciting ways to keep anyone from hurting her in any way ever. Perhaps preemptively.

Vampires aren't as quick to experience personality changes as humans are. They take their time, they bury their emotions and weaknesses. They appear to move on, looking to the rest of the world as they always did. Don't let the fact that your character is either (1) afraid of or (2) vindictive towards this character impact party cohesion.

For the time being, anyways.