The validity of the PSV's "attach rate" with regard to comparisons in NPD sales threads is questionable.
If one included PSN/XBL downloadable titles and minis and classics and whatnot, the PS3 and Xbox 360 would presumably be much higher than their 8-9 retail titles a piece.
I'd hazard a guess that Vita's digital to retail ratio for full games is better than the PS3/360s, We saw a 20-35% digital share on some 3DS titles in the past year or so, and Vita's install base is even more "core gamer" due to its tiny size and emphasis on indie titles. No way to prove anything without concrete numbers though.