Ok, it can be mods. It can be also other things as well. The best thing I suggest is to make a save right before you get ready to face Vek. Then get out of the game completely, pop the cd for a moment, then pop it back in, and start the game again. Then make a list of mods. This sounds like either a mod conflict Or a game problem. The save and restart of the game may work, if it does, that's the problem. Also, try cleaning the cd, as well. If it is not that, then it is a mod problem. Making a list of mods installed, and also looking at the readmes of the ones that have them, and other things, may help you figure out what's going on. Start simple.
When everything else fails, CHEAT!