When I ask if there's anything I can help with, every NPC in Castle Volkihar sends me to speak with Vingalmo but the only dialogue option I can get from him is "Why can't Orthjolf be trusted?"

Is there any fix/work-around to this, or am I screwed out of continuing the side/radiant quest lines now?

This bug has been around since the beta and I'm just wondering if anybody figured out a way to progress past it somehow.

Nope there's not.

make sure that you dont already have a quest given to you by the vampires

I don't.

Hopefully the team at BGS has already seen this posted in the beta forums and here as well, but just in case they haven't, regarding this bug:

My playtime is 132 hours on this specific character.

Size of save is 12 MB.

Skyrim and Dawnguard are both installed and saved to an external flashdrive rather than my HDD.

Problem occurs in, obviously, Castle Volkihar when speaking to Vingalmo for more radiant side quests. There is no way to progress the side quest-line for the Vampire side and every single NPC refers me to Vingalmo no matter how long I wait for or try to find a workaround for it.

I'm on the Seeking Disclosure and Chasing Echoes questlines in the DLC on this specific character that is plagued with the bug. The side quests I've completed are both Ancient Amulets and Rings of Night/Blood Magic quests, as well as one single Blood Chalice upgrade quest. I've also gotten one Deceiving the Herd quest, one quest to change an NPC into a Vampire, and two quests to kill a Master Vampire.

Just giving some of the info they asked for when archiving bugs on the beta forums, as I'm sure this info is still useful to them in some way.