The Wii U had an abysmal January, only selling 50,000 units in the US. The Vita did even worse, possibly only selling around 35,000 units in the US. One of these systems will survive. One of them won't.
There's a brief window at the beginning of a console's life cycle, after the launch, that the purchasers end their initial honeymoon with the console, look around and ask, "OK, so now what?" If the console has a steady stream of games on the way, the purchasers continue onwards happily. If there isn't, a bit of a rebellion starts to form.
Let's look back at the 3DS. The 3DS launched with a crappy original lineup, and afterwards the cupboard was bare for months. The best game from the original launch was Ghost Recon: Shadow Wars, and it remained the best game on the system for months afterwards. Nintendo had to respond by dropping the price on the unit and offering early adopters free games to mollify them.
With time the games came in, and the 3DS now has a surprisingly robust lineup, with plenty on the way. Of course, now the 3DS sells well and the early hand-wringing over whether or not a system with glasses-free 3D would work is mostly gone.
The PS2 experienced something similar back in the day. If you'll remember, the launch lineup for the PS2 wasn't very good,. There were just a few games: Tekken Tag Tournament, Fantavision, and a few other middling games. However, two things propped up the system: 1) Downwards compatibility and 2) The ability to play DVDs, since DVD players were pretty expensive at the time. That led to people buying the system because it was a cheap DVD player and they could still play their PS1 games on it. After that, the games started coming in and the PS2 sold like wildfire, becoming arguably the best system of that generation.
All right, so the Wii U has underwhelming sales right now. That's because so far there are only about two or three really good games for it. There's no hype about anything new coming out. After some initial excitement, everything appears quiet.
However, if there's one thing Nintendo knows how to do, it's make games that sell systems. They just need two or three exciting titles to turn this around. In a year or so, we might be talking about the Wii U's robust library of great games in the same way that we talk about the 3DS. It's still early in the Wii U's lifespan, so there's plenty of time to turn it around, build excitement, and continue onward.
The Vita doesn't have that luxury, though. The initial excitement over the spectacular quality of the hardware has died down. (And make no mistake, the Vita is a great machine.) They just dropped the price in Japan, usually a hotbed of portable gaming.
Most importantly, though, the release schedule for the Vita is empty. What's on the horizon for Vita purchasers? Best case scenario, a few games tossed in its direction that will be middling-to-good with a couple of great ones. Worst case scenario, lazy ports of Madden.
The ceiling for both of these systems has been lowered somewhat. The Wii U isn't going to trounce the competition like the Wii did, but it can hope for performance like the 360 and PS3 this generation: Solid systems that did good business and turned a profit. The basement is still the same: Something comparable to the Nintendo 64, a good system that tried some new things and wasn't as good as you remember it.
With the Vita, the ceiling has been lowered drastically. The best that it can hope for is something akin to half the sales as the PSP. The basement? Not as bad as the Nokia N-Gage, which sold only three million units worldwide. Still, with only about five million units for the Vita, it's not far behind.