My Club Nintendo Luigi 3DS case. (Of course) it holds a 3DS just fine, with some room to spare. It does not hold a PS Vita. I could maybe squeeze the Vita in there well enough to barely close the zipper, but I don’t think it’s a good idea, and I’m not sure it would work.
Speaking of things barely fitting in this case, that little pocket holds a maximum of one game — and it takes some work to get that one game in there! Of course, there’s nothing stopping me from just tossing hella games into the case with the 3DS, so no big deal.
In conclusion, this was way more fun to get in the mail than my gas bill. If you want to sport a tiny optical illusion of a hat to carry your system around in, it’s available through Club Nintendo (North America) for 450 Coins.