I'm vegetarian (well I do eat fish rarely, so a pescatarian). My dog is pescatarian because she has to be. She doesn't handle meat or poultry very well at all. Dogs can easily be pescatarian. They can also easily be ovovegetarian (egg is a great protein source for pups and it is okay to use it in place of meat). They can techinically be vegan also and it would be impossible to say that vegan dogs live shorter lives. I don't think that info is even available. I'm pretty sure Purina H/A is vegan. It would be pretty difficult to get this diet correct, but it's still possible. Whether or not an owner wants to force their dog to eat the same way they do is up to them. I'm fine making my dog be a pescatarian or even an ovovegetarian. Anything beyond that and I don't think I'd want to force that on my dog...
BTW, I know rx diets aren't appealing, but when owners get backed into a corner and need to give it a try, it suddenly isn't so bad anymore. Happened to me... I don't like Iams either (and now Innova), but the meat people eat comes from way crueler places. No difference really.
Anyway, your dog may not need an rx diet, but if you keep trying things and they aren't working, it is best to give it and get the GI upset taken care of. That becomes a quality of life issue and it is kinder to just give a diet that works for the dog.