People like vendors, photographers, musicians, bartenders, etc., not only don't expect to mingle with the guests but would generally prefer to NOT mingle with guests. I can't speak for ALL vendors, but in my musical days everybody in the band brought a brown bag lunch and other service people did likewise. Sometimes the food service had "vendor meals" for us, thoughtfully provided by the event's hosts, but we brought food of our own just in case.
I recall one mother of the bride warning us to NOT mingle with the guests. Our smarty pants guitar player quipped "In that case, you get a 20% discount" and cost us all money.