I had this problem too. The only solution I found that worked best for me was saving the project under two different names. This way all the media and events would stay the same.
Under the first project file I deleted all the files(pics) that kept disappearing when I rendered. I then rendered the video track as an HD avi file (uncompressed).
Next, I opened the second project file and deleted all the events that actually rendered in my first project. This left only the events that never rendered. I then import my HD avi file onto a timeline and then rendered the new complete file. Granted, its not ideal, but it’s not any worse than transcoding.
My biggest problem is that this version of Vegas has way too many bugs and I cannot render mp4 files for blu-ray.