I suppose you don't have the original files in your video dir (otherwise will be simple) so you can do 2 things:

1) create yourself the thumbnails and upload the to Vshare Thumb dir.

2) Copy the flv file to Vshare "Video" dir and assign it to the corresponding video in sql database. Then go to to admin console of vshare and upon video click "create thumbnails".

Remember, BACKUP BEFORE DOING ANY CHANGE! ;)