How do I restrict who can buy from a vendor geographically?

Hello,

That kind of restriction should be made on the product and not the vendor; Except if you let your vendors configure their own shipping methods.

Regarding the HikaShop features, I think that the best would be to use the warehouse sytem ; thanks to that you can have shipping methods which won't authorize some geographical regions.
Otherwise, you can use the different Shipping method restriction and use the category filter.
For more details, don't hesitate to post a new message in the HikaShop section of the forum.

Regarding the "vendor location filter" plugin includes in HikaMarket, that plugin is mostly to filter the product listing depending a postcode provided by the customer (or its main shipping address).
It won't allows to perform or check restrictions on the shipping methods ; it is more a plugin to help regarding local delivery features.

Kind regards,