merchants that should assist in combat?
 

There were many vendor npcs that were assisting their primary faction in combat. This is wrong and I fixed them for next patch.

It made me wonder if there are any caveats to this. Should any specific vendors assist in combat?

If you're attacking city guards, all NPC's in the area should assist and beat you to death. Just my two cents.

I don't recall any vendor's assisting in combat