I didn't have any probs with Vegas, but I was just doing analog caps, not HD. Googling showed me several (undated) hits which talked about having to use a DeckLink card...
Code:
http://www.digitalproducer.com/articles/viewarticle.jsp?id=31944
has a lengthy setup guide (with pretty pichurs) from a big fan of Vegas, you might find some tips there.