So I'm up to Fort Locke and I've all but finished the above quest. Problem is, however much I click on Vallis Anton to confront him nothing happens. I went into the crypt, beat the Necromancer, found all three men, and returned to the Fort with no other problems.

edit:

If it helps, Tann never joined my party, but I've heard he's supposed to at some point, when I talk to him in the crypt he just says what he said before I found all three men.

How does he join the party? Do you have to talk to him after finding all three? Does he join after beating the Necromancer? After leaving the crypt?

No, you speak to him after defeating the dark priest, before you find his men. He joins you to find them.

I went back through the crypt to where you talk to him, but he doesn't join the party after the dialogue.

There is a bug (or used to be; don't know if it's been fixed) whereby if you talk to the missing soldier (NE of Tann) that's findable before speaking to Tann, the quest loop was broken, because Tann expects you to find 3 men, but the one you've just spoken to disappears. Have you spoken to this guy before Tann?

I just finished this quest yesterday. While exploring the crypt I had talked to Tann first (just found him 1st by luck) then found the 3 soldiers he wants to find. Maybe you "broke" that quest by finding the soldiers before Tann. You may need to go back to an earlier save. You may know this already but use the quicksave (f12) A LOT! Lost a few battles because characters would sometimes "get stuck" & not fight or run off & start another fight while I was currently in another battle. The qs feature is nice since it only eats up about 5 seconds. Hope this helps.....