Do you presume to know what does and does not cause a tremor?
"but feeling a 'tremor' seems like a very odd thing to say."
As opposed to what? As far as the films go, the only thing that causes anything more than a tremor is the destruction of a planet.
"but the last time he felt a tremor in the Force was around Obi Wan? What is the Emperor, chopped liver?"
I believe he's referring to the distinctiveness of the tremor. The fourth version of the script states this:
Quote
A tremor in The Force. But the only
time I've felt it such as this was in
the presence of my old master.
"Wouldn't that get to be a bit distracting, as much as Obi Wan and Anakin saw each other, that everytime Obi Wan walked into the room, there was a tremor in the Force? "
Keep in mind that, in ANH, Vader doesn't say "The Force is strong with this one" until immediately after Obi-wan talks to Luke. It wasn't Luke that Vader was sensing - it was a tremor in the Force (or, more specifically, Obi-wan, who is now one with the Force.)
I might also add that Vader knew which hallway Obi-wan would be taking in ANH. Funny that he didn't go to the hallway that his children (whom you think he sensed) were in, and together, no less.
Not to mention that Force sensing isn't an exact art, and Force presences can be hidden (Ben didn't sense Vader in the hallway, Palps didn't sense Luke at Endor, and Qui-gon and Obi-wan didn't sense Maul on Naboo.)