Nope. That's the short version. Of course, it depends on what you mean by natural. If you mean those parts of us that have gotten buried in overcivilization, the things we were built to do but for which we're given only poor substitutes, the losses that are killing us now, no, veganism isn't natural.
You know, I think nothing makes vegetarianism and veganism look sillier than phony arguments. People are built to be omnivores, plain and simple. We're opportunistic eaters. The old argument about teeth is just wrong, because have both teeth for tearing meat and teeth for grinding vegetables. The end. You can find a more detailed discussion here.
So if you appeal to biology to say that avoiding animal food is natural, you're going to seem uninformed at best, and foolish or fanatical at worst. Not great publicity.
If you look to archaeology too, there's no evidence that hunter-gatherers ate only vegetables, and in fact it seems like they ate a lot of meat. It's generally believed that the Clovis people hunted mammoths to extinction, handling that natural resource as carelessly as we handle oil today, though the position is controversial. Nevertheless, they certainly did hunt, as did other hunter-gatherers.
Even humans that have been stereotyped as carnivorous cavemen, it turns out, ate more of a mix. Neanderthals, it seems, ate their veggies. Sorry guys, there's also no evidence of empty beer cans, sports jerseys, or issues of Maxim magazine at digs. I'm not giving quarter to diehard steak-eaters either.
See, look at this guy. He's more sensitive than that. The selfish-but-lovable-oaf type in sitcoms isn't any more natural than anyone else. Don't blame this poor fellow for the decadence of your empire.
Anyhow, it seems that people have generally just been people, and that we've eaten in roughly the same way. We eat everything we can.
So I guess you can't call veganism natural.
But what is natural, really? I mean, is it possible for people, as animals, to do anything outside of nature? Are we not skyscraper-building animals? Are we not polluting, warring, blogging animals? Are we not animals that create decadent empires, and moronic, egotistical TV characters, and the fictional smart-mouthed termagants that love them?
There is something in our nature that seeks higher values, that produces moralities and philosophies, that strives for something better. In this, as in many other things, we're unique as creatures on Earth. We can conceive a bigger picture, we can respond to conscience, we can choose to limit ourselves, we can make choices for the sake of something other than mere survival.
So I guess being vegan is natural after all. It's just not natural like a paper napkin with brown dye in it, sporting a big recycling logo. It's natural as yet another strange thing that the human animal does.