I have the same setup as you. Two computer monitors and one HD TV. As you point out, many other programs recognize this TV, even Avid and some other video player programs, but not Vegas. Too bad…
My current setting is “2 displays, Independent mode + 1 feature display”. Vegas only see the two regular computer monitors though. I hoped there was a way to make Vegas output to the third monitor, but it seems as I have to live with the two for now.