My husband was a vegan when I married him— I was a vegetarian, but ate eggs and dairy. He had no body odor whatsoever. Never wore deodorant, skipped showers sometimes, even on a workday. Ate onions and garlic liberally, and nothing. He was the eighth wonder of the world.
Then he got deployed to Iraq. He managed to stay away from meat, but he was a big guy, well over 6′, who just couldn’t get enough calories without eggs and dairy.
After he returned, I revised to extremely carefully selected eggs and dairy that were environmentally friendly and cruelty-free as possible (and kosher, so no rennet, gelatin, fertilized etc.), and he continued to include these foods in his diet.
And while he didn’t develop a strong odor all the time, any time he ate garlic, he stank to high heaven for about 12 hours, from several feet away. Onions weren’t as bad, but he’d have awful breath for several hours, and nothing covered it.
My point is, it could be diet, but you could eliminate lots of foods, and not find “the key,” because it could be the interplay of different things.
Trying some of the diets recommended by those who have “been there” is your best place to start, but if that doesn’t do the trick, you could try an elimination diet. This is where you eat just a couple of things for a few days— things generally known not to trigger reactions, like brown rice, undressed baked potatoes, and such (Google “elimination diet”), then one by one you add food— one a day, usually, until you get a reaction.
If a strong suspect (ie, garlic) is added early, but isn’t a trigger, then something you wouldn’t suspect, like skim milk appears to be a trigger, so you eliminate it, and a few days later, gelatin triggers as well, consider the interplay of garlic and animal products. That’s just an example— you need to keep a diary to look generally for problem interactions.
If you narrow it down to something like garlic and animal products, you make a choice— avoid one or the other if you will be around other people a lot. Spending the night by yourself binge-watching Perry Mason (which you just discovered)? Time to order garlic chicken.
Keep in mind that garlic and animal products is just an example from my experience. It might be something totally different, like raw greens and yeast products.