Sony has resorted to removing games from the PlayStation Store in order to thwart PS Vita's piracy.
To be more precise, vulnerabilities in the deleted games were by the Vita Half-Byte Loader to allow homebrew applications to run on PS Vita.
Vita Half-Byte Loader is capable of running homebrew games such as SNES and Game Boy emulators, but it cannot be used to run any pirated PSP or PS Vita games.
Instead of releasing a patch that closes the loophole used by the boot loader, Sony decided to delete the vulnerable games from the PlayStation Store, even though this has prevented some users from downloading the games after paying for them.
It is reasonable to expect that Sony will return the games to the PlayStation Store after patching them or patching the PS Vita firmware.
As of now, Motorsorm: Arctic Edge and Everybody's Tennis are pulled off the PlayStation Store.