Have all your followers wait somewhere were they can't see what you're doing, go kill whoever you want, then come back for your group. They'll be none the wiser. Enjoy.
Darkwalker is right, but some followers do not care when you kill someone. It is nice to keep these ones, find out which they are. Also, when you kill someone, your followers will just have a slightly lower reaction to you, generally it doesn't matter. Who is Vincent anyway?
smart, i'm playing a persuasion character atm, but last time i played the party turned up dead half way into the place, so i haven't even bothered with the ripper quest. Tah tho, tis usefull info dat.
Hmm... characters like Geoffrey or Torian Kel don't care of what you do, or the most loyal: Virgil! When Magnus used to go "Hey, I don't understand why we're attacking such a descent type...", Virgil didn't doubt on killing those innocent conspirative gnomes... I hate Tarant and its gnomes with their lab-ogres. Now, Vincent can't be motivated to attack you? Guessing that NO, I'd say that ignore what your followers say and just kill him. If they get too upset, read the "Followers Gift" thread and there's the way to boost their reaction.
I would make your followers wait even if killing Vincent wouldn't upset them. It'll botch the quest if they attack him, so leave them somewhere else, regardless of whether or not they'd care.
Besides when Vincent is attacked with anything different than the right dagger he changes into a demon and fights back.
If you don't want your followers attacking him, just hit F5 and they'll do nothing for the rest of the battle.