Was the stain visible in the photos or did it only really show up once you had the sweater in hand? Because that changes everything honestly.
Technically you're right that you're within the 14-day window, but honestly Vinted's system isn't as straightforward as regular websites. If the stain was noticeable in the photos, even if it's small, the seller will probably argue that it wasn't hidden. But if it was really imperceptible or if the photos are poor quality, then you actually have a solid argument. What matters is the description vs reality, not just your change of mind about the style.
I bought pants that were supposed to be "lightly worn" and when they arrived there were dye stains or whatever on one leg. The seller first refused the return saying it wasn't their problem, but I opened a dispute with Vinted and provided clear photos. They sided with me and I was able to return them. The key is being precise in your request: explain exactly where the stain is, send well-lit photos of the area, and compare it with what they'd announced. If you just say "I don't like it," they won't do anything, but if you say "item described as new with a stain not mentioned," you have more leverage. Try talking to the seller first by showing them the photos, otherwise open a dispute on the platform directly.