Wen,
The JFBConnect Demo site is the best example you'll see. While VirtueMart isn't installed on that site, the registration flow for it is the same. The demo site is setup with the Full Registration flow, so after you click the Login With Facebook button, you'll be brought to a page where you can log in with an existing Joomla user account (to associate it to your Facebook one), or the ability to register a new account. If you choose the 2nd option, you simply have to fill out your desired username and password as well as any Virtuemart fields that you may ask of the user that aren't being imported.
On the demo site, we import the user's location and likes and a bunch of other fields automatically into JomSocial and Community Builder, but the effect is the same with Virtuemart.
The other registration flow of automatic registration (used on sourcecoast.com) where a Joomla account is created automatically is not available when you're using Virtemart integration. That's because to create a Virtuemart user and add them to a shopper group, their full profile must be filled out. Since many times there are fields that can't be imported from Facebook (mainly zipcode) that are required by Virtuemart, you need to use the 2-step registration flow of JFBConnect to get that information.
Hope that all makes sense, but if you have other questions, just let us know. We also have a 30-day money back guarantee for just these types of testing purposes so you can see if JFBConnect is really an ideal fit for your site!
Thanks,
Alex