Kassey here from the Shopify Guru Team! I definitely see where you're coming from! What you can do is, create collections for each vendor, then use the tax override method to set specific taxes per collection! You can see how to do tax overrides here!
I hope this helps, but if you have any further questions - be sure to reach out to Shopify support!
This solution no longer works and you now have to create a manual collection for each tax override. So if you have a vendor with a 1000 products, good luck creating a manual collection for them if they are different brands.
Not sure why this changed, but when it did, it deleted some of my vendor collections from the menu where i choose a tax override and somehow kept other vendor collections, even though they are set up the same.