If you have a collection with a handle that matches the handle of whatever is in the vendor field for a product. For instance:-

Vendor name of “Brand X” uses the handle “brand-x” as long as you have a collection that exists at /collections/brand-x then the code will look at the products in that collection and find any with the tag.

Sounds like the products you are trying to link probably exist in different “colour” collections, so won’t be found. You could switch to use the product type instead.