Hi,
You can have a manual activation and let your vendor pay a specific product in order to let you activate their account.
You can also have a "automatic activation" linked to a specific Joomla user group.
Then, thanks to HikaShop Business and his plugin "usergroup after purchase", a customer can buy a product which will add him into a special "vendor" group and automatically, his vendor account will be create/activate.
Once the vendor account activate, the user will be able to create his product, change his vendor profile, etc.
Regards,