Firstly, some backstory:
The party has recently released a large group of prisoners from a fortress orbiting Baator, and escaped with the freed prisoners on an Astral Ship of some sort. As is appropriate, they offered the poor souls places on their crew, and the prisoners accepted. However, one of the former prisoners is a Vampire played by a player who's character recently fell (it was a significant and awesome death, I promise). Partway into the journey, the player reveals to me that, while in captivity, he used his vampiric powers to bite and convert all of the other crew members.
I am woefully ignorant in Essentials, and am unprepared for this development. So far, I have just warned the party to keep the ship out of sunlight (is there even sunlight in the Astral Sea?), but if there is any official ruling on a PC Vampire spreading his loathsome malady, I would appreciate the enlightenment.