Please don't link mercola here; it's a quack site, and not reliable.
http://www.quackwatch.org/11Ind/mercola.html
You really can't trust anything he says, so please don't link there as a source to anything. Like a broken clock is right twice a day, he will occasionally be right, but the vast majority of his positions are based on an agenda of pandering to his audience and selling things, not on science.
See the vegan cats site FAQ:
http://www.vegancats.com/veganfaq.php#1070
Is it true that vegancats.com is telling some people to feed their cats meat?
Yes. After much soul-searching, we have decided to change our official recommedations for certain cats.
Urinary tract problems are fairly common in cats, and because of the relative acidity of meat to vegetable protein, some vegan cats suffer from them.
Although we have been giving advice for some time on how these problems can be minimized while still feeding a completely vegan diet, we have found that many of our customers have not been following this advice. It is imperitive to feed your cat a vegan diet properly for optimal health, as not doing so may jeopardize your cat's well-being (as is the case with improper feeding on any other diet - ideal nutrition is a necessity for optimal health!)
Unless you are very committed to following the advice outlined on our site by feeding your cat a properly balanced diet as per the information below, we recommend that you mitigate the risk of urinary tract problems by feeding males cats only a 25-75% vegan diet and females a 50-100% vegan diet. Of course, many cats (both make and female) will thrive on a simple vegan diet and never have any complicatons from urinary tract infections or crystal formations, you need to be prepared to adjust the diet accordingly for cats who may be prone to such issues.[...]
Dogs can pretty easily eat vegan; go with a recipe, and give them some supplements to be safe.
Female cats can eat formulated vegan diets pretty easily too (most will have no problem).
Male cats may need a little bit of meat to adjust urine pH if they have urinary tract issues, or careful monitoring of urine pH.
Here are a couple cat and dog foods that are generally regarded as well formulated:
http://www.vegepet.com/
http://www.amipetfood.com/en
Most dogs will have no problem on these, except possibly very large breeds as puppies (this is a rare exception, where they may need extra creatine or other supplements)
Most cats will also have no problems on these diets.