Most people would agree that cats are carnivores, and need meat to survive. Dogs are omnivores. They can survive on a carefully planned, nutritious plant-based diet, though many vegetarians still feed their dogs at least some meat or meat-based dog food. Not that this matters much, but dog food generally has much less meat as a percentage than cat food does. (PS: If you have a dog, and would and would like to find out more about a vegetarian diet for dogs, check out this article, or this one.)
An important thing to remember here is that humans (at least most of us) have a choice when it comes to eating meat or not eating meat. Animals don't have this choice. While we should respect the lives of all sentient beings, we should not mimic the behavior of these animals, such as the carnivorous diet of cats for example. I believe that were given freedom of choice so as to be able to choose a compassionate path, and that, for humans, vegetarianism and especially veganism represent progress.
That said, helping carnivorous animals survive is not a bad thing. Look at the organizations that work so hard to preserve species of birds and wild animals around the world. More often than not, the animals in these refuges require fresh meat for survival. On a more local level, like many people, I care about our shelter pets. I have done what I can to help encourage people to adopt, and to spay and neuter their dogs and cats. Encouraging the spaying and neutering of pets is something that I believe everyone who cares about farm animals should get involved in, since humanely cutting down on the population of unwanted pets will go a long way to minimize the amount of meat that is used to feed these animals, thereby saving many lives.
When all's said and done, it all boils down to how comfortable you are with having a pet that requires meat to survive. Like I said, for people who truly empathize with the plight of all animals, it's often a very hard choice to make. Some vegetarians/vegans will not have pets for this reason alone, and I can respect that. I also respect and understand the other side of the argument, one that recognizes that carnivorous animals sometimes also need to be saved.