Answer from Dr. Armaiti May, vegan dog specialist & vet.

I recommend gradually increasing the proportion of vegan food, while still feeding the former diet that the dog is accustomed to, over a period of 1-2 weeks, until the vegan food comprises most or all of the dog's diet. This gradual approach should minimize the occurrence of gastrointestinal upset (vomiting and/or diarrhea).

If the dog is going to be switched back to the other diet shortly after being fostered, however, it might be beneficial to include some small amount of it along with the vegan food so that he/she doesn't suffer from gastrointestinal upset once put back on the diet after being off it for a long period of time. Sometimes adding pumpkin or squash in small amounts can help keep the stools a normal consistency if there is a dietary-induced change in stool quality. If the dog eats something that causes vomiting, a period of fasting (with veterinary guidance) and/or bland diet of tofu and rice may be indicated for a few days until the dog's system normalizes again.