If it really is a straight 'dinner' gig, then I think it should be all instrumental. 'Dinner' usually means EATING and being involved in conversation (socializing) with the people at your table. Singing distracts from that. Some, or most, of the people at the table will want to listen to the singer, especially if they're really good-----or really bad ). In either case, it detracts from the 'socializing' much more than background instrumentals (which only serve to create a festive environment). However, none of the above applies if it's a casino-style 'show', where the emphasis is on the show and dinner is included just to justify the (outlandish) cost of the show. I've played both types and IMO, the pure 'dinner' type gigs, which usually feature only instrumentals, could be served just as well with a 'jukebox' ('elevator music' piped in over the house PA). JMO, though. YMMV.

chas