Hello,
The current vendor selector type will list all published vendors.
Adding a feature to add "vendor groups" will not change that.
It would be more convenient to improve the "vendor selector type" by adding a filter on Joomla group than creating a new kind of vendor structuration and add the same kind of filter to the selector type.
But, even if we add that kind of filter, you do want to have that filter dynamic depending the product.
The vendor selection is based on a custom field, which can be an "order field" or an "item field".
If it is a "order field", you will have issues because you might have a conflict between products and cannot select just one vendor for the entire order.
If it is an "item field", you will have issue because a custom field do not have the information of the product.
You can access to all the other fields (and their values) but you have no access to the "parent" object (user, order, item, product...)
It would require a lot of modifications in the HikaShop and HikaMarket core to introduce a way to do such.
Otherwise, you can override in the product page the way than the item custom field is displayed ; instead of displaying the official vendor selection field, you skip its display and perform your display of a dropdown. In the product page context you will have access to the product data and you could determine which vendor you want in that list.
Afterwards, thanks to a custom plugin, you can also add a function to verify that the cart data is correct before the creation of an order and the vendor selector system will do the rest by making the vendor association and process the fees.
I do understand what you want to achieve and I'm sorry there is no built-in feature to have such kind of control.
The vendor selector was design for "sales representative" needs, with the features and possibilities that HikaShop fields offers.
Regards,