1) He can do this, yes! Let's suppose he captures a neonate because that is the most plausible. They'll tell him that they've been sent by their sires and their Lord Morningstar to attack the carnival members attending the party - they'll say that they'd been tracking the carnival across the multiverse until a few days before the party, at which point their magical connection was destroyed. They don't know how Morningstar figured out where the carnival was going to be, except that it had something to do with a powerful contact outside of the vampire's domain, who had a mutual interest in fucking with the carnival.

This tip came in a couple days ago. The neonates and their sires were sent in with the main objective of capturing the supervisors, and turning any other carnival randos they could find. It was revenge for all of the vampires they killed during the last confrontation Morningstar's forces had with the carnival - particularly the Nightrider's crew starting shit by assassinating some of them on the vampire's home planet.

This vampire will also plead that Lambert doesn't kill her in exchange for this information, but. Well.

2) You don't really have to because Lambert is probably going to kill them. I have arbitrarily decided that they are female and that they will plead for their life, but beyond that you can assume what you want.

3) THAT'S UP TO HIM.