Vita sales are massively undertracked as we know that there are an awful lot more digital sales on the system than you normally get on a console. Wasn't there something from Sony that said as much as 50% of software sales on Vita are digital?
I wonder how many sales Gravity Rush and Uncharted haven't registered due to them being claimed via PS+.
Also 3DS might have 16 500k sellers, are any of them also on Vita? It's generally a common consensus that while the Vita has great games, it doesn't have enough "big" exclusives. It would be interesting to see accurate sales for the multi format 3DS/Vita releases.
There are quite a few Vita titles have sold better than their 3DS counterparts despite the much smaller userbase. Surely at 40m to 6m using extrapolation a 3DS game selling 500k is the equivalent of a Vita game selling 75k on a sales to users ratio.