I dunno guys I just cant imagine them putting in a system where drops are predetermined...but I can see why this may be true with vanities.

Another example:

killing random mobs...drops are spread out randomly among your team. Ive gotten like 5 drops in a row before and I definitely didn't kill all the mobs that dropped them.

However, when you get a vanity drop, I am almost certain the one who delivers the finishing hit, the one who gets the kill tally, gets the vanity drop.
I've only ever seen two vanities, comedy ad tragedy, and one of them was mine, and I definitely killed that enemy.

This direct drop could mean that it is in fact NOT a rolled item, as it seems that not only are regular drops rolled, but the person who receives them are as well.