Curiously clicked on the very last link at random and this is an excerpt of what I am reading:

In 1994, People for the Ethical Treatment of Animals (PETA) reported results of a systematic survey of the health of 300 vegetarian dogs sourced from 33 states within the U.S. and Canada via PETA’s newsletter [54,55]. Dogs ranged in age from young puppies to 19 years old, and included a wide range of breeds, males and females, both neutered and entire. Of these, 65.3% (196/300) were vegan, with the remaining 34.7% (104/300) simply vegetarian. They had been maintained on these diets for anywhere from less than two, to over nine years, with an average of 5.7 years. The precise diets used, and their level of nutritional adequacy, are unknown. Over 80% of dogs maintained on vegan or vegetarian diets for 50% to 100% of their lifetimes were reported as being in good to excellent health (Figure 1).

Bolded the absolutely laughable parts. The other "studies" mentioned in that article are about the same: Voluntary participation in a non-compensated study (leads to self-selection bias), owners simply reporting everything is fine (how do they know? perceived fur shininess is baloney), blood tests inconclusive.

The best you can pull from that article is that switching to a vegetarian diet for your cat/dog is likely not immediately endangering them. So it's more "do so at your own risk" instead of "it's recommended" and I don't see how that is in the best interest of your pet, I'm sorry.