You will find it basically impossible to keep a vegan diet in any branch of the service. And you can probably forget about being even vegetarian if you're deployed into a combat zone.
(No, I'm not in the service. One of my brothers just retired from the Army, though, and I've asked him about this before. He says there are a few vegetarian MREs, but he hasn't seen conclusively that there are vegan ones. And the dining hall options on post are VERY limited in terms of nutritionally sound vegetarian options.)
I'm also in contact with several Marines currently serving in Afghanistan and Iraq. The incredibly lucky ones are in decent quarters that don't smell miserable, have offices that have AC and a fridge and a freezer. But they still rely on the PX to buy snacks and stuff and they sell out quickly, and being choosy isn't an option. The lucky ones shower once a week and get one hot meal a day. The not-so-lucky ones basically live off beef jerky, MREs, warm energy drinks, and talking to each other. Some are low on everything from tactical equipment to food. The reality is we even have hungry deployed Marines who aren't even getting ENOUGH food to sustain them, never mind being able to be picky about what they eat.
Even if you've got someone sending regular care packages with vegan stuff, you can't rely on that. I send care packages to these guys and sometimes their mail is held up for weeks at a time.
You can make a commitment to the animals. And you can make a commitment to your country. But I assure you, when push comes to shove, the only legally binding one is to your country. If you haven't enlisted yet and you feel giving up veganism would be a no-go, then I urge you to reconsider making this HUGE service commitment. The two don't really mesh.