As a website owner, it is recommended to add Vendor Products to different collections yourself so that you can control the user experience on your website. However, in case you still want Vendors to control their listing, you can do it using 2 ways
1. Create Automatic Collections in Shopify using “Vendor name” as a condition
a.This will help you automatically assign any product belonging to a Vendor to selected Collections.
2. Create Automatic Collections in Shopify using “tags” as a condition
a.This will help any vendor add “tags” to products which will in turn decide which Collections they get added to