DO NOT UPGRADE/MIGRATE A LIVE SITE
Any VirtueMart update should be made on a backup copy of your site - never on a live site. This allows you to ensure that changes in the VirtueMart code or templating do not affect your site functions, and if they do, to apply any necessary template changes.
Virtuemart 1.1 and 2.0 are COMPLETELY different generations. It is a migration, not an upgrade.
The same with J1.5 and J2.5
Akeeba Backup is a must for joomla
https://www.akeebabackup.com/download.html
Threads to see
http://forum.virtuemart.net/index.php?topic=78844.0
Upgrade Instructions
http://forum.virtuemart.net/index.php?topic=95236.0
Same Result/ Different Instructions
http://forum.virtuemart.net/index.php?topic=95389.msg313121#msg313121