I know of a former diver who went vegan after being vegetarian for many years, apparently. His cat died at approx. 15 and spent her last few years on a vegan diet created by a vet. Both the dry and canned versions from this vet's line contained enough nutrients/supplements to compensate for the lack of meat--apparently. She died from kidney failure, IIRC, which is not at all uncommon in cats, and extreme stress as evidenced by her back leg that she'd gnawed down well past the fur in one spot. I have no doubt that she was miserable eating that slop... day after day after day... He's got a dog now who is on the same regimen, but he doesn't touch my heart so no need to comment on him.
I loved that kitty and wished I'd known more about cats at that time, as I would've asked for her to come live with me--and would've transitioned her back to the correct diet consisting of primarily raw food. Fortunately, I live in a progressive province that has a whole whack of raw food companies, so no lack of variety, as my current feline companion could attest to.
"I sincerely wonder if any of the vegans or vegetarians who read my columns feed their dogs or cats a vegan or vegetarian diet. I understand that those of you who have bunny rabbits most likely do. If so, how do you justify letting your fine furry friend eat what you won't?"
I think the question should be more along the lines of...
How do you justify imposing your lifestyle choices on a living creature that would reject them otherwise?