My boy Rafa was skinny too, but not as skinny as your Dobby, the solution for us was to give him goat's milk as well it really helped him to put on weight in addition to his normal food. You say you feed him twice a day, I wonder how much food as all the recommendations on dog food quantities printed on underestimate a Vizsla's energy calorie burn off, so they need more food for their age than most other dogs are recommended. You will struggle to overfeed a Vizsla, they will leave excess food, unlike a labrador so I would give Dobby more food too.