I bought a PS Vita used, help me get the old PSN account off
Here's a link on Gamespot to help you accomplish what you want. I've included the important text below.
Power off the system and hold down the power button, right trigger, and playstation button at the same time and then a list of options will pop up. You can choose to restore the PS Vita, and format the memory card.
There is another comment a few down from this one that confirms this and was apparently what Sony support told them to do.