When the die do you think Molag Bal claims their souls too? Is there anything in lore that supports that idea?

Werewolves would be far more alligned to Hicrine, who claims their souls however is unknown at least to me. I think you need to actively support a Daedra in order to have them claim your soul, and not all werewolves actively support Hicrine, some just become werewolves by accident...

Werewolves are beasts of (I believe) Hircine's creation. He uses them in his hunts and anyone, no matter by accident or not, is under his "thumb" I guess you could say.

In the companions quests of Skyrim, I won't spoil anything, but a member does not like the fact that he has beast blood and openly says he won't make it to Soverengard (sp) if he still has the blood because he is indeed a beast of Hircine.

I really don't have a clue about Bal though. Not much knowledge on the Daedra yet.

Actually he isn't sure, he believes he won't be allowed in Sovngard, but he basis that believe on absolutely nothing.

I don't want to spoil anything but I'll say this. Keep playing and you'll find out that he was right.

This. Hircine is more attached to werewolves than Molag Bal is to vampires in a way. Most of the time we see Molag Bal dealing with mortals and whatnot. Hircine always has to do with werewolves. In Daggerfall, Bloodmoon and Skyrim and even the novel: Infernal City.

The Cyrodiil vampires made a pact with Clavicus Vile to become stealthy, so it's possible that they end up spending time with him.

Wouldn't be surprised at all if that was the condition on their little blending in with humanity perk. Vile does love collecting souls after all.

True. I always imagined the drawbacks of that perk was the backwards "Starve and get more powerful but look monstrous, and feed to get weaker, but blend in" thing. Vile does love collecting souls, but vampires are the creation of Molag Bal. But we all know that vampires, if they had to chose, would be the realm of Clavicus Vile. As bad as it may be to spend eternity there, it would be worse to hang around the Prince of sixual violation.