One of my two original players is an evil vampire spawn/cleric. My three other players were introduced into the campaign and they chose to be non-evil, all of them pivoting around shades of neutrality. I allowed a vampiric/undead character because my campaign setting is often twisted and the world is rife with corruption. Since the Vampire had been in the story/world the longest, I decided to have all of the players meet up with him (they all joined at roughly the same time) and join to hopefully form a team of adventurers, which was what everyone wanted. So I wrote in a line of prophecy that made it reasonable for the rest of the players to follow an NPC to a dungeon where they'd be able to not only have a good fight, get loot and XP, but also be able to meet with the Vampire and go on some adventure or other together.
My problem? It's been a dozen sessions since they met up, the vampire has fled and tried to escape every attempt at having even as much as a conversation with the other player characters, and my game is slowly being sundered.
Not everyone is going to agree with a sadistic vampire's ideals and his agenda to annihilate all life, but I figured if I gave the players a common enemy to fight, they could either set aside their differences or learn how to capitalize on them and form a stronger party. Yet this has not been the case. It's been a wild game of "chase the vampire" and every time the players have a chance to unite, the vampire feels threatened and runs away again. To rectify the situation I've had to loosen up, but it's only made it worse. Instead of just running around a desert, it's turned into an interplanar teleporting spree of unimaginable scale that makes me cringe, and I feel wholly responsible because it's the NPCs, typically, that have these powers (most of the PCs are not magic-based). I made a common enemy for them but he's too powerful and they managed to fight him way before I anticipated.
Based on a very long discussion I had with a player after our session last night, I realized that the only way for this group of PCs to collaborate is either to destroy the vampire or run two separate campaigns, one just for the vampire and one for the others. And I don't want to do either of those things because it'd be unfair for the group, impossible for me to work into my real-life schedule, and boring. In fact, at times it feels like I'm already running two separate campaigns, because in order for it to "work" sometimes the turn of a single PC lasts an hour or more. And it drives me crazy that I can't figure out how to stop it.
The key issue is that my player is so attached to his vampire PC that it is visibly causing him, as a human being, real psychological damage/trauma. Whenever his power is challenged, he begins to get very stressed. If there is even the hint of a threat to his character, he gets anxious. Two sessions ago he almost had a panic attack when his PC was under the effect of Control Undead (perhaps a mistake on my part). I get frequent phone calls asking when the next session is going to be, but sometimes it becomes unbearable pestering (we play once a week), and it's all this anxious panic over the idea that his PC might be defeated. I feel like a horrific DM, and like everything in my campaign revolves around a single PC -- it's totally unfair for everyone else, and I personally am not having much fun forcing events to happen just to try and remedy this insanity. If the vampire is defeated, I fear everything will be too far gone, the campaign will collapse, and I will have psychologically harmed my best friend.