Hello,
I have experience with Joomla 1.5 and Virtuemart. Some separators in the products infos may cause the problems when uploaded.
Do you need to repair the old products in the site, or just clearing the products and a new smooth upload will be enough?
Access to your site database and ftp(or any other read/write) access to the files will be at great help.
Best Regards,
Strahil