Assume you don't know the vendor, you only met though wedding planning, no doubt the vendor would eat at the reception but would you allow the vendor to bring a plus1? Another scenario is the vendor is 2 or more staff. Would you even bother extending a plus 1 invitation? It could easily add up to 10-15 extra guests for the reception which are vendors. Personally I would say no only because the vendors are here to do work, ie, setup, makeup, photography, etc. They are not here to entertain their plus1 who knows no one at your wedding. But I am not sure what is the norm.