Yes, this can be made simple with a subform. The main form is based on the table on the "one" side of the relationship and the subform is based on the table on the "many" side of the relationship.
Since you have a many to many this means you can add data to the VendorProducts table in two manners:
~Main form based on Vendor table and then a subform based on VendorProducts where Vendor is held constant (in main form) and you select a product in the subform.
~Main form based on Product table and subform based on VendorProducts where Product is held constant (in main form) and you select a Vendor in the subform.
Check this link out for details: