Jeremy,
Our Virtuemart plugin copies a lot of the functionality from the user_class from VM into our plugin and then applies some minor modifications to it to adhere to our user creation process. You can see the file in the /plugins/jfbcprofiles/virtuemart.php. Specifically, if you search for "shopper" and "shopper_group_id", you'll see it being set in a few different places. Depending on the field that you have setup in VM, you should be able to check for that value in our file to see if a certain code was entered and then change the group's setting where necessary.
My description is a little vague because I don't know your specific circumstances. However, if you need more help getting a value or changing the group, just let us know what field specifically you change about the user and what input field you look for. Then, we can see if we can help provide a little more info on how to accomplish what you're looking for.
Hope that helps get you started,
Alex