Make Joomla menu links to VirtueMart for registered access only, especially the link to VirtueMart Home Page.
- you can leave a public link to VirtueMart Account Maintenance if you want to provide a registration form for wholesalers and set Joomla's user manager to require admin approval for registration
- or publish the Joomla login module if you want to manually create users.
- VirtueMart modules such as search and category menu can be set to display only to registered logged in users.
- You can add another step and make the products only displayed for the wholesale shopper group.
- Also enable in the VirtueMart config "enable checkout only for registered".