Do dhampirs in Pathfinder become vampires when they die? I thought so but I don't know where I got that impression. Is this true in Golarion? If so, what about nosferatu-spawn dhampirs? Do they become nosferatu when they die?
Nope—just like mortals, when a dhampir dies it's dead. (Check the last sentence of the Bestiary 2 entry: "Although polluted by undeath, dhampirs do grow old and die, aging at a rate similar to elves.")
That said, there's nothing stopping a dhampir from acquiring vampirism through traditional means.
While Renie is entirely correct, it would make a very interesting campaign element!
Ah, OK. I was re-reading some Ravenloft materials recently. I must have read it there and got mixed up. Thanks.
Nothing to be ashamed of! I, for one, might use exactly that for campaign traits. :)