Hi, and welcome to EBN. Opie is very cute. Congratulations. You're on the right track switching Opie to a better quality food. Beneful and Purina are not good choices for any dog, due to the poor quality and the fillers, and additives in the food. Have you been on the Home Page to read the Dog Food Ratings Forum? It lists all the dog food brands and lists then from a 6 Star food down to a 1 Star food. You want to choose a brand that is a 4 Star or better, for quality and nutrition. Examples of these foods are Fromm's, Blue Buffalo, Wellness, Arcana, and Orijen. You mentioned that you switched to Blue Buffalo, and this is a great food. The next thing you want to look at is the source of the protein. Many bullies are allergic or sensitive to chicken and grains. You may want to try a different protein and a grain free food. Every dog is different in what they tolerate, it's really a trial and error process to find a food that they tolerate. When you did the food switches, did you do a slow transition over two weeks, or did you switch all at once? Changing foods too quickly can also cause tummy upsets and diarrhea. A puppy usually is fed more frequent meals throughout the day and the usual amount is 1 cup of kibble 3 times a day. They are usually fed puppy food until the age of 6 or 7 months, and then can be switched to an adult formula dog food, as well as the afternoon meal can be eliminated, and you can feed him 1 cup of kibble twice a day. I don't have any experience with the Vegan dog food, but again if you are going to try it, do a slow transition to the new food. Good luck, I hope you find a food that Opie tolerates. Please keep us posted on how he does.