Hi Matthew C-
I believe the vet is telling you not to use Vit c or cranberry supplements with the prescription food because it will over acidify the pH of your dog’s urine. That situation could cause the other type of stones. Struvite stones are formed in alkaline urine. The prescription foods help acidify the urine and increase thirst in your pup. More water and moisture as Anon mentioned will help dilute the urine.
Struvite stones in dogs are often associated with infections as you mentioned. One of my cats had a blockage and ended up in the emergency clinic for three days due to Struvite crystals. I now feed him mostly canned food, keep his litterbox clean, and have a water fountain available. I hope you can find something that works also. Good luck!
