I liked Vaughn when he first showed up and the CIA basically showed Sydney that she didn't know squat about just how big SD-6 actually was, but that was about the extent of it. When they started the oh-so-obvious creation of UST, my eyes rolled so often that it was hard to get them to stop.

With Vaughn, it's almost like he enjoys being the "suffering and supportive" one. Besides making him "sensitive", it allows him to angst and become the center of attention. He seems to like being there - to the point of shoehorning himself in at times. I think he likes the dynamic of having his wife and girlfriend right there in front of him - not to mention the whole thing of Sydney's having been gone (so to speak), so that her feelings are still rooted in the past and still centered on him.