Hello,
The vendor page is a view in HikaMarket ; so it is right related to the add-on and not to HikaShop itself (by default you do not have vendor pages in HikaShop).
You can customized the HikaMarket vendor page like you can customize any view.
To display the vendor terms into a popup ; you can use some code that you'll find in the view "checkoutmarket / terms".
The idea is to create a popup link (or button) and give a specific link which will display the vendor terms automatically.
hikamarket::completeLink('vendor&task=terms&cid=' . $vendor_id, true)
You can also add some custom code to display or not the link if the vendor do not have any "vendor_terms" configured.
Regards,