The new one won't fly. The original 1.4 would work if you replaced certain files, although you dind't get many of the bennies that the standalone version gave you (because you're limited by the Vegas interface). But the new 1.41 won't render, so i went back to the originals.
I use Main Concept 1.04.01 which works well for MPEG2 encoding.
I haven't tried to use it directly from Vegas, I just render out the file as a DV AVI, and then open that with MC.
You can't use the MainConcept standalone encoder directly from Vegas. Rendering to AVI from Vegas and opening it in the MC encoder is how you'll have to do it.