Hi,
I don't see any HikaMarket license attached to your account, in the future please use the account which is linked to your license.
The solution will depend on how your subscription system is working.
HikaShop have a plugin "group after purchase" which allow you to change the user-group when a product is bought.
So if a user want to become a vendor and buy the special "subscription" product, the plugin will put him into a special joomla group and you will be able to make a difference between classical users and your vendors.
Otherwise, HIkaMarket have a plugin which change the user group of the vendor administrator account.
When a user become a vendor administrator, HikaMarket (thanks to the plugin) will change the user group and, like the previous solution, you will be able to make the difference.
After that, I don't understand why people will have to have two logins.
Regards,