I have a memory card I have been using with my account. Is it possible to deactivate and format my memory card, and set it up for another PSN account?
Yes, it is.
First, follow the deactivation steps to deactivate the account (this is optional and can be done in a PC later on, but it's quicker to do it in the console).
Then, go to Settings > Format > Reset the PS Vita System.
After the system is reset and formatted, you can set up another account on it.