I can't help you with the wmv files created by Microsoft Community Clips (I get the same result - video conversion works fine but no thumbnails). Part of this is likely caused by the fact that wmv files created with this software are only 10 frames a second, whereas decent quality wmvs are normally 29.97 frames per second.

Regarding the mv4 files, I have found that if I rename my .m4v files to .mp4 before uploading (or via FTP if I'm uploading to the import folder), Vshare handles them fine.