Well, it depends on where you live, where you're going, and what you're doing on vacation. For example, if you're vacationing in or nearby a large city, you would have many vegan options. You should be able to duck into just about any grocery store and put together something quickly. As long as your company/companions understand you have slightly different needs. And local Vegan restaurants can be as easy to find as a quick click of the mouse
https://www.happycow.net/ It does help to do a bit of research into where you're traveling.
So personally, I plan ahead, and map out spots where I'll be able to stop and stock up on supplies. Even knowing when you’ll next be able to visit a supermarket next can be an invaluable step towards making sure you have access to fresh fruit and veggies. (Look for dark green salad mixes in the produce section – they often have pre-washed spinach and other ‘instant’ salads that can provide some much-needed nutrition on the go. Pre-packaged spirulina smoothies can be found in certain grocery stores as well.)
As Amber suggested, trail mixes, dried fruits, and also dried veggies, granola bars, power bars, cliff bars, dark chocolate, and ginger chews, and veggie drink mixes for on-the-go smoothies.
Don't forget your supplements, if you take any...B12, etc. Make sure you bring along a few condiments that will make your foods taste better, nutritional yeast, soy or hot sauce, mustard, etc. Often when on the road, I'll pack a small chef's knife and cutting board in my car, which gives me a few more options when eating out of my vehicle via grocery stores.
Some fruits make perfect travel snacks, like bananas, apples, oranges, etc. Anything that comes with it's own packaging that doesn't require refrigeration. Even just a can of beans can make a filling and nutritious meal. You can rinse them right in the can, and eat them from the can. While this doesn't sound so appetizing, it is still satiating and can work in a tough situation.
There's lots of great vegan travel blogs as well, I'd suggest checking out as many as you can.
https://www.veganfoodquest.com/
https://www.vegansociety.com/shop/books/lifestyle-books/vegan-passport