Whether it's doing well or not doesn't necessarily bother me. When the 3DS came out I could care less. I saw it in Target, tried it out, and was not impressed. Other than the new Kid Icarus game, I've had no real desire to buy a 3DS. I loved the regular DS and the GBA. Both were awesome systems with awesome games. At this point, I see absolutely no reason to buy a 3DS.
Other than being hindered by awful, just awful, marketing, the Vita should be doing really well. It has access to a ton of the PSP library (I've bought a ton of PSP games both some I've played before and some I hadn't. Definitely worth their purchase prices), the screen is gorgeous, the touch screen is responsive, decent/good battery life, and there are plenty of good games for it. Wipeout, HSG, Lumines, Plants vs. Zombies...Granted these are basically retreads, but they are definitely good retreads. The new Mortal Kombat is a very good handheld fighting game and I'd never even thought about buying a handheld fighting game before because typically they just don't work. MK Vita works well. VT4 is also a very fun game.
Granted, there isn't a ton of excitement about the Vita due to slow releases (it's cool Gravity Rush is almost out) and next to no marketing (at least where I live). If people gave the Vita a chance and accepted it for what it is, I'm sure plenty of people would be quite surprised...Unfortunately, it seems people would rather hope for its demise thus partially creating their own self-fulling prophecy.
In short, buy a Vita and I doubt you'll be disappointed in the short-term..In the long term????...That's up to Sony and other developers. With a new Sly Cooper and JSR coming up at least that's something to hope for.