Not sure why no one else has mentioned this yet..but the easiest way to prevent food aggression is to be with them, touching them, while they are eating. We've started this young with all of our puppies. While they eat, we pet them and touch them, their paws, the food, their muzzle..we keep our hands in contact with them and their food while they eat. We did this for the first few weeks..and now none of our dogs are protective over their food..which is great as we have kids running around now!
Since your dog is older..I wouldn't recommend immediately sticking your hand in his bowl! Start by petting him for a few moments while he is eating/growling(haha)..and increase the time you are doing it each time he eats. Once he will eat and ignore your hand on him, put your hand on the side of the bowl and leave it there. Don't move it. After he is okay with that, lay your hand in the food..by then, he should be much more tolerant.
We rescued our Boxer and she had serious food aggression when we got her..we did the above and now she is fine with anyone going near her bowl while eating.
Definitely don't spray your dog with vinegar..so many things can be taught without those types of training methods. You just need the patience to follow through.
Remember to start off easy...good luck!
Hailey-Boxer dob 3/26/06
Kylie-GSD dob 8/24/07
Blitzen-GSD dob 2/4/13
Bandit + Tigger-our cats