I'm pulling the following out of my ass but I don't believe Vita is taking 3DS sales so much as there's an offset between the two.
Basically, with its current library and price, 3DS has peaked in its sales and it appears that most new releases appeal mostly to the current installed base. Note that even though it has peaked, it keeps steady sales. I'm not saying it's dying, just that 50k+ weeks need to be jumpstarted, they don't happen just because the system is there.
On the other hand, Vita is coming back from a bad bout of leprosy and after much struggling, it has hit a point where the combination of its price and library is more enticing than it used to be. A steady stream of solid, if not ground breaking, titles has helped a lot in establishing that library. It's not doing gangbusters but it's very much alive, just selling modestly.
The biggest risk for Vita IMO is that most titles address the same niche and once the console has exhausted that niche, I'm not too sure where it can go sales wise. That wouldn't affect software sales too much though.