Lock The PS Vita Screen
You can password protect your PS Vita; you will need to set a 4 digit passcode. This passcode will be used to unlock your PS Vita when turning it on or coming back from standby mode.
Tap Security -> Screen Lock -> type in a 4-digit number. Do not forget this number.
Tap the checkbox so that a checkmark appears. Now the PS Vita will require this passcode whenever you turn it on or come out of standby mode.