Have a quick look for a larger size before owning up maybe??? It's truly the thought that counts.
Yeah that’s the thing w buying clothes it’s a 50/50 chance that it won’t fit and will need exchanging. You know her best. Even though she’s financially comfortable do you think she’d judge if you admitted it was on Vinted? Like is she overall an empathetic person. If she is empathetic and understanding in general, I would just come clean. There’s nothing wrong w what you did. I would honestly not care if someone bought me something on Vinted, there’s a factory outlet near me that sells last seasons’ clothes for dirt cheap and even though they’re “new”, exchanging sizes is a PITA because of limited stock. So it’s the same thing really. Buying from an outlet, or buying “new w tags” but unused.
Im sure she wouldn’t mind ! Just because she’s financially comfortable doesn’t mean she wouldn’t understand your situation. Obviously I don’t know her but I’m sure she’d appreciate the effort you’ve done to to get her present regardless of price xx