I haven't used Sony Vegas before, but iirc it is some pretty high end media stuff. I wouldn't expect the E-450 to run it very well at all.
Maybe others who have used the processor, the program, or both will have a better answer, though.
TBH, I don't think I would try to salvage much of anything from the current PC. I would try to sell it for a fair price and then buy completely new stuff.