My two Vitas I always tested in tandem by rubberbanding their sticks. Even if the game don't have a function on the right stick (like Shovel Knight), it keeps the devices from dimming down the displays after a few seconds of no input:
The Vita with its gorgeous OLED screen + 3G is 5 years old, I got in at launch day. Its brightness range is much more narrow than on any other of my devices, on minimum brightness it is still brighter than my Vita Slim or my 3DS XL on medium brightness. Unfortunately that also shows in the tests... you only gain a few minutes when setting the brightness to the minimum... "Killzone Mercenary" and "Uncharted Golden Abyss" as "worst case scenarios" both were around 3 hours, no matter which brightness ofr if Wifi and 3G were on or off:
"Limbo" and "Shovel Knight" as "best case scenarios" had much better results. Efficiency in PSP-mode and PS1-mode were also quite good.
My 2 year old Vita Slim has a much wider range both in brightness and in battery life. I can play 5 hours "Uncharted: Golden Abyss" with all bells and whistles (or "Killzone Mercenary" with half brightness) and up to 10 hours Shovel Knight (Vita) or "Crash Bandicoot 3" (PS1) with half brightness or , "GTA Vice City Stories" (PSP) with low brightness: