(translation from Japanese by me, apologies if it doesn't sound natural in English).
Q: How many PSN account can be set up on a Vita system?
A: Only 1 account. If you want to to use a different account, you need to format the system to factory settings.
So there you go. Forget having multiple accounts for your brother or your gf. Forget having multiple accounts for each region. Just cancel your preorder, it's not worth it anymore #trolling
I'll add the original Japanese phrasing, in case someone wants to try to do a better job at translating than me...