I don't see it happening.
Firstly, the 3DS has a 15mil lead over the Vita (which will likely be nearer 15.5mil come Vita's launch). That's a lot to make up, and puts Sony on the back foot.
Second is brand recognition. The public are becoming wise to the 3DS on the market, especially since the Christmas rush. This combined with big games coming/ recently come out (2 Mario, Resi, Kid Icarus to name a few), word of mouth from people who have one and a £150 price point makes the 3DS far more appealing to the average consumer than a relatively unknown £240 PSP with no real info., unless you frequent gaming sites/magazines.
Third is the image. Parents will buy their kids a 3DS over a Vita because of the ads, and the knowledge that Mario and co. won't hurt anyone. The Vita is an unknown quantity; as such parents would choose the cheaper, more reliable (in their eyes) console over the new kid on the block, and save anywhere between £65 and £100 in the process.
Finally is dev support. With a lukewarm reception to say the least in Japan, and a massive lead for the 3DS, not to mention a large number of truly massive games coming for the 3DS (MH4, Animal Crossing coming soon, and Smash Bros., Zelda in the pipeline), sales are not looking likely to slow. Were the Vita to get a similarly weak reception in the rest of the world (which, let's face it, is quite possible), then it could be all too easy for projects to be 'mysteriously' cancelled or delayed, just like the 3DS' launch. Were this to happen, Vita support would diminish further and further, and support would dry out further, in an ever-growing whirlpool of abandonment for the system.
Due to the App market, I doubt either console will hit their previous console's numbers. This is what I expect in 6 years time:
3DS- 132mil
Vita- 65mil







