[MENTION=6673096]FiddleSticks96[/MENTION], it would seem you find the D&D alignment system too rigid for your liking and wish to house rule it to be more flexible. That's fine, but you should make sure your players are informed of this in no uncertain terms.
Why? Why should his players have the meta-information that vampires could be good? If this DM want to have a good aligned vampire, he should. He doesn't have to, and shouldn't tell his players that vampires could be good, and instead have the actions of the vampire NPC show that she is good. Characters don't (well, shouldn't) know the MM, but might have some basic knowledge of monsters in their world. If there is only one, or only a handful of these good vampires in the world, why should, and how could, the characters (and thus the players) know that those exceptions exist?
As if characters would stand around arguing:
Character 1: "Well, she seems nice"
Character 2: "And she helped us, I like her"
Character 3: "No, the MM says vampires are
always evil, so we should kill her. Good vampires cannot exist"
Character 1,2: "Let me see that (players look-up vampire in MM), yeah, you're right, lets kill her"
Above is meta-gaming (or as I like to call it, cheating). And although this isn't a huge issue in most tabletop RPG's (it is in LARP), it should be discouraged.
If the characters would stand around arguing like this, it would be better:
Character 1: "Well, she seems nice"
Character 2: "And she helped us, I like her"
Character 3: "No, don't you know vampires are
always evil, so we should kill her. Good vampires don't exist"
Character 1: "Yeah, good point, let's kill her"
Character 2: "But ... I like her. I don't wanna kill her"
Think of it this way, if you were to sit down to play Monopoly with a group of people, then were designated the banker, everyone would expect you to give them $200 when they passed Go. If you personally decided that $200 was too much and that for this game you would only hand out $100, then as soon as someone passed Go, an issue would arise.
Nowhere in the DMG of Monopoly does it say that all rules in this game can be changed at will by the DM (or banker). That's the difference.
House rules that apply to everyone need to be stated clearly ahead of time, especially where they pertain to important player decisions. You have a paladin on a quest to prove himself worthy after losing his abilities and then you introduce a very morally ambiguous character into the equation but you are not following the RAW for alignment.
Important
player decisions. This game isn't about players decisions, it's about character decisions. Player make certain decisions, because it's what their character would do. Alignment isn't an in-game mechanic, just like all other stats. Did you ever have your character say:
"Well, this boulder looks heavy, I don't think I can push it, my Strength modifier is -1. Well, trying once can't hurt, let's roll. *rolls* Booyah, natural 20, eat that, stupid boulder."
I sure hope not, because if you did, you must have missed the letter R of RPG.
Back to character decisions. Their characters should decide, based on their (the characters that is, not the players) previous experiences and on the current situations, being, a nice vampire lady protected them. I'm not saying it would be wrong if they still killed her for being 'evil', but this choice should not be based on player knowledge, or system mechanics.
TL-DR: Whatever the MM says, DM has the final say, players shouldn't cheat with MM information.