I have heard of some vegan dogs living near a friend of mine. I need to see what they are actually eating. It is possible they could survive but to do it right may cost a lot or else the dogs may get malnourished or have other issues in later life due to diet.
I'm sure if you feed a dog on potatoes all it's life it would be healthy enough.
I feed my dogs Raw and kibble 50/50, with rice, fish and potatoes occasionally. I feel a high energy dog needs good food and raw meat meat provides this.
I sometimes wonder, many dogs survive on kibble 80%Maize 20%'animal protein', Would they really be that bad off on veg only.
I've seen the benefits of raw and for over all health, I think a dog should be fed a balanced diet which gives the dog all it needs for a healthy long life.