They were removed by Sony
The two exploitable games are back on the PSN but they have been patched by Sony.
Sony removed some weeks ago MotorStorm Arctic Edge and Everybody's Tennis from Psn because they were used by hackers to run unsigned code on the Playstation Vita with the homebrew loader VHBL. Now both games are fixed and back on PSN, so if you purchased them in the past they should appear again in your download list.
Obviously VHBL doesn't work now with this version of the games but Wololo is already working in a new version of VHBL compatible with firmware 1.67, and it will use a vulnerability of another Psp game, not revealed yet.
NEWS SOURCE: http://wololo.net/wagic/2012/04/18/motorstorm-and-everybodys-tennis-are-back-on-the-psn/
Our thanks to 'Robe24' for this news story!