Here is video of the problem I am having, captured by someone else with the same issue:
http://www.youtube.com/watch?v=CASTEgAcpw0

I am supposed to activate the dwarven mechanism at the top of the stairs on the right to release the hot oil, but the valve is turned 90 degrees in the wrong direction (compared to videos I have seen where this scene works), and there is simply no way for me to activate the mechanism. This means there is simply an endless barage of Kaedweni soldiers and I can't move forward in the game. Please help. Am I missing something or is this a bug?

For others with this same problem, there is a cumbersome workaround discussed here:

http://en.thewitcher.com/forum/index.php?/topic/29333-game-broken-vergen-besieged/page__st__40__gopid__805917

Basically, you have to reinstall from the original, patch to 2.0 (not EE), then complete the Besieged at Vergen section, then update again to EE. I would like to see a patch of some sort though.

Thanks for linking to this. I have the same issue. :(

Yeah, pretty much anyone using a carryover save from the original has a good chance of encountering this bug. I eventually decided to backup my saves, reinstall the original, load my save at "Besieged at Vergen", complete that section, save again, then reinstall the Enhanced Edition.

It was a pain in the ass, but it worked. Thankfully I had a backup of the 2.0 version from GOG on my external harddrive and a backup of the EE version, so I didn't have to do a bunch of extra downloading. Hopefully you are able to do something similar.