I Dm'd my first game recently and the teams cleric (Who followed a good god) ended up swearing allegiance to a vampire mistress for free enchanted gear, she now uses his head as a way to travel around throughout the day, while he still follows his original god. Needless to say the entire party was wondering what the hell he was thinking. We ended up having to drop him down two alignments in the course of like 3 in game hours.
No his character knew it was against his religion (The religion he follows is completely against any form of undead, and the only reason he didn't fully attack on sight is because he still isnt fully trained.) but thought eh whatever gloves of +2 strength are worth it. He then praised himself (in character) for not being a paladin so he didnt lose any abilities.
This whole campaign works within an original universe plus Dm's ruling. The party hasn't figured it out yet but they will later learn that cleric abilities within this universe aren't actually from their god and its just fancy magic. So that rule isn't taken into affect.
Assuming 3.5 D&D because fuck it
A cleric loses his powers if he grossly violates his cleric code. If his god disliked undead he no longer can use his powers until he atones for his sins.
Next time he prepares spells he gets nothing, and he feels his sins upom his back.
The vampire can give his evil spells though if he wants that! You should offer him that choice.
The dwarf is trying to get back into contact with his family (He left the dwarven stronghold because he didn't want to follow traditions) So im planning to have a little subquest involving that.
The Mage is going to have to face his peoples history being a sham.
The bard is going to have to learn what being a bard is actually about.
The main plot is a simple (albeit not to orignal) story of uniting multiple provinces against a demon lord. It mostly takes a backseat to the different factions and their leaders problems that need covering before they can do anything to help.