I can play where certain creatures are evil, and we kill them. I can play where I don't just kill orcs and vampires on sight. But I've really got to know upfront what the rules are for this game, and I'm going to be very unhappy if my paladin gets punished if I feel you changed the rules.
I'm not saying the paladin should be punished if he killed this vampire, just because they are both good. This is a fundamental problem with the DnD alignment system, which I will not elaborate on further, so not to hijack this thread, but the choice should be made on a character level and not on a player level. This whole good vampire attacking the barbarian thing is a bit of a gray area, so here a nice black and white example.
A paladin, named Bob, walks through town. There he meets this beautiful, but somewhat pale lass named Alice. Bob and Alice fall in love, and he asks her to marry him. She says yes! On the night of their wedding, she admitted to him that she is a vampire. Bob slays her on the spot. When he is arrested by the town guards (Alice was very much loved by the townsfolk) he says: "She was a vampire, and the MM says they're all evil. I'm a paladin, I have to slay evil beings."
(Note: Bob's creed isn't that specific, he just has to purge evil and what not. Alice is the only vampire in this world that is good, the rest is totally evil all the way).
If Bob's player would now complain to the DM that he's being punished for doing something that would be a good deed according to the rules, I hope we would all agree that Bob's player is a total idiot.
The DM had no reason to 'publish' the 'house-rule' that vampires could be good, because none of the characters know this. If I was said DM, I'd keep the vampire alignment thing very much a secret, because otherwise I'd give incentive to meta-gaming.