The AI options in control settings appears to be bugged or I don't understand them. I unchecked everything except healing and also checked option "use quick disciplines only" and removed from Wilhelm's quick bar everything except feeding and healing. But even though he still uses all kinds of disciplines whenever sees an enemy. It pisses me off because he's always short on blood and I need to give him the flasks and whatnot. I can't imagine playing this game with 4 characters party if it will looks like that.

Also, where I can find some merchant? I'm after monastery dungeon and have pockets full of junk as well as some unidentified items and haven't seen one yet.

I seem to remember having to reset the AI settings every time I reloaded the game for it to stay in effect. As far as merchants go, if it's the monastery in the first part of the game that you just got out of, you should have Unorna (I think that's her name) to sell/buy vampire specific items. After you cross the bridge from the main city, I think it's the door to the right. For armor/weapons, you have to go back to the main part of the city where the smith is.

Later on, you'll get access to a couple of more shops after the party.