I agree that mixing it with food is the best bet. You'll want to try to make it as pleasant as possible for him. On the other hand, some dogs just won't take it. My grandmother's Keeshond is like that with her liquid arthritis medicine. She's a snobby eater and shares a food area with another dog. She also refuses to open her mouth or take treats. Granted, I could probably control her food and make her hungry enough to eat it, but that's a lot of work and the dog won't like you any better for that than for squirting liquid in his mouth.
With the keeshond, I simply take the needle-less syringe and put the tip near her back teeth, holding her nose up. I squirt it and it goes between her teeth. She has to swallow. She locks her jaw and won't open her mouth for pills and the like, so this is the best option I have available. If you can get him to take it through trickery, that's best. This is also an option.
Lastly, your vet said it's okay for him to take vitamins, but did he say they were necessary? The best option you probably have is to buy a high quality dog food. That should do a much better job than giving a lower quality food and trying to balance it with liquid vitamins. On the other hand, if you're already giving a high quality dog food, then he probably doesn't need the additional vitamins.