Nothing will happen. Speaking as a dog owner who had this very discussion with my vet yesterday, I can assure you, that had the dog eaten a whole bottle, nothing much would happen, except maybe an upset stomach.
The reason I had the discussion was that my breeder had tyold me to give the pup 500mg of vitamin-c, daily. I thought this to be BS and asked the vet. He agreed and said dogs don’t need vitamins, provided you give them a proper diet. They can also make their own vitamin-c, as opposed to humans, so no need for supplements. “What if I give him vitamins?” said I. “Well, it won’t harm him, but it won’t do much good either” said vet.