What happened
It's 2 weeks later and since some people wanted an update, here is what happened.
The party was able to retrieve the artifact, but were very leery of it since they were afraid that was what was making the vampires "good". With the artifact in hand, they returned to the lead vampire, Milosh, who seemed unaffected, until they showed it to him. They watched as Milosh appeared to return to his old evil self. Thinking quickly, they hid the rod from his view and he returned to his "good" side.
He told them his clan were compelled to the monastery to retrieve the artifact and seeing it brought back that compulsion. He then insisted they leave as soon as possible.
As they exited the catacombs, the monks had attacked and the vampires were not going to win without help. Once more, great RP trying to decide what to do. Basically, there were three choices:
1. Side with the monks, who might consider them thieves for taking the artifact.
2. Side with the vampires, killing the monks, but allowed "something special, good vampires" to continue to exist
3. Running away and not getting involved...I would have worked hard to avoid letting this happen.
A brief attempt at diplomacy momentarily halted combat as the 2 leaders met, however, the monk leader was feigning interest in peace and blatantly stabbed Milosh with a hidden dagger, reigniting combat.
The party rogue immediately went into action as did the dragonborn fighter and Wizard. I've been experimenting with half HP and double damage for the monsters to speed up combat and make it more interesting. Rolling 4d10s for damage does make for some hard hits against 16th level characters. Anyway, through excellent tactics and use of bursts and blasts, the party decimated the monks, including the leader, in their general area.
The rest of the monks stopped combat at the death of their leader and his second, promising never to return in a hostile manner.
Even after the victory, the Warlord who was still distrustful, and the Cleric, who was almost hostile, would not accept the thanks nor hospitality of the vampires.
Once this was all done, I did a show of hands. Three of my five players were convinced the vampires would turn on them. One was on the fence most of the night and one believed the vampires were good. Anytime I can upset player expectations, I consider it a job well done.