Hello Josh,
JoomSEF (and its VM extension) takes as a first part of the SEF URL the name of the main menu item that points to virtuemart. If that is not found then "virtuemart" is used by default.
So you should be able to modify this by renaming the menu item pointing to your VirtueMart component. We also plan to allow configuring this as an option, but that is not possiblem yet. Now you can either used the menu way or it must be hardcoded in the VM extension.
Please note that after changing the menu item item, you will need to delete the existing auto-generated SEF URLs
Best regards,
ARTIO Support Team