March in Sikkim, huh? Sounds like a good time to visit. For vegan street food in Gangtok, you'll want to focus on momos and thukpa, but you absolutely have to specify "veg" and then "no paneer, no cheese, no butter, no egg." It's like a boss battle, but you can win. Many smaller stalls will use a little oil, so you're generally safe there. Look for places selling sabji-roti; the sabji (vegetable curry) is often vegan, but again, confirm no dairy. Expect to pay around INR 50-100 per plate.
In Pelling, it gets a bit trickier. Street food is less prevalent. Your best bet for cheap vegan options will be small dhabas or eateries that cater to local workers. Ask for dal (lentil soup) and rice, or a simple vegetable curry. Always double-check on butter/ghee. Don't expect a lot of variety, but you can usually find something to fill up on for INR 80-150.
Lachen is super remote, like a pilgrimage to a sacred anime location. Food options are limited and mostly in guesthouses. It's not a street food paradise. Talk to your guesthouse owner in advance. They're usually very accommodating and can make a simple vegan dal-bhaat or vegetable stir-fry if you request it, but it won't be as cheap as street food in Gangtok. Budget around INR 200-300 per meal there. Your best bet for budget in Lachen is to carry some dry snacks.