So far, Durkon has only interacted with his old friends and his enemies. Being turned into an evil-aligned vampire won't affect how he deals with enemies, except perhaps to make him more ruthless (as in breaking Zz'tdri's neck). How he interacts with his old friends...
Well, put this way. The old Durkon was a very Lawful character, with a lot of scruples- so much so, for example, that he would not willingly fight Miko
here along with the rest of the Order. Roy is Lawful Good but was willing to resist Miko anyway- but Durkon is so motivated by his alignment, in particular by the idea that resisting Miko is an unlawful act, that he stays out of the fight.
Now, becoming a vampire reverses Durkon's moral alignment (good/evil), but not his ethical alignment (lawful/chaotic). So he is
still a very lawful person, which means he will take old debts and responsibilities seriously, and that he will still feel strongly bound to deal with the overarching threat to the world.
He isn't even the first Lawful Evil person to be willing to help stop Xykon; Tarquin was quite willing to assist Elan. Indeed, Tarquin only switched sides and turned against the Order of the Stick when Nale was able to convince him that it would serve Tarquin's long range goals. That's typical for an orderly character... but what long range goal does Durkon now have, that would override his decision to help the Order beat Xykon?
As he put it, "world still needs savin'." So far, Durkon has been presented with very linear moral choices where being Evil rather than Good doesn't really affect what he does very much.
We'll get a better sense for how (or if) Durkon's personality has changed when we see him in a less restricted environment, one where he deals with more normal NPCs and social rules.
