Here's a little trick to play game demos Such as Gravity Daze on your USA account Vita, I came across it today and thought you guys might want to know.
Demos tend to have less security than games making it easy to play other regions demos on a different region PSN account
P.S. if you want to play Gravity daze demo in english make a hong kong PSN and download the english demo. It takes about 15 minutes to accomplish minus the time to download demos.
If they dont have the same security measures as regular games cant you just download said demo on the ps3 and transfer it
Edit : tried this method and it wont work
You clearly didn't watch the video, it has to be done on the vita system itself
I have tried it and It does indeed work. I am playing Gravity daZe as we speak. Add me on psn and you'll be able to see I am playing it on this account which is PSN usa
I'm sure what you did may have worked if your vitas memory card was linked to your japanese and american account but since you probably haven't even attempted the measure I put up. Do not bother commenting on the post if you have not even given the tutorial a chance. I just thought it's be nice to share a load of demos that arent released on the USA psn. I'm going to add you to show this is indeed working