That's an odd issue but can be fixed pretty easily by selecting the customer from the search tab, clicking Open and then kicking off the YouTube render again. The file to be uploaded to YouTube does not exist which means it's been deleted manually, or the upload has finished and the software deleted it.
Looking slim on YouTube possibly means a different rendering engine could be used. Try rendering a short video and uploading it to YouTube outside TandemVids. Then use the same rendering engine.
The default rendering engines I choose for YouTube do not exist on Vegas 10e so the rendering may be using something completely unsuited to YouTube uploads. If you go to the YouTube rendering tab and press F1 and then scroll to the end of that help file section, it shows a good custom rendering engine to use.