Wildlife Conservation and Protection

While it's hard to verify sites aren't AI nowadays, I did some digging and found a few blogs with advice that seemed legit. Here is the article that most closely matches your situation: https://gardeninguru.com/how-to-save-a-crushed-snail-whose-shell-is-broken/

If the snail is still there, I would suggest getting a safe 'ramp' to put in front of it, like a sturdy leaf, playing card, or piece of cardboard, placing it in front of the snail to let it crawl onto it, and then using that to transfer it to your 'snail hospital' shoe box. From there, research such as this article suggests that the main thing the snail needs is calcium, a damp environment, and time. You do not need to attempt to repair the shells, just give them time to heal. There is a video in the article I linked about repairing a crack in a snail shell with the membrane of an egg and some superglue, but it sounds like the damage here may be such that it would be better not to mess with the shell vs. the extensive handling of trying to reconstruct it.

For calcium, this article suggests very finely crushed eggshells (like, powdered in a blender, because otherwise it can be sharp). Offer a shallow dish of water and some leafy greens, and change it daily until it recovers. I'm not sure what the best vegan alternative would be if you are vegan; as a vegan, my plan in your shoes would probably be to just ask around for eggshells from my neighbors. I might also try high-calcium leafy greens like spinach. EDIT: Oh, that site you linked suggests natural chalk in another article, so that's an option for you.

For housing, while an aquarium or tank would be best, it appears that a cardboard shoe box with some ventilation holes in the lid will work. Add some moss and leaves if you can, and mist at least daily with a spray bottle for moisture, or as often as it takes to keep it damp.

As with any attempt to care for an injured wild animal, brace yourself for the very real chance that the little guy will not survive. Thank you so much for trying to help, even if it doesn't work out.