Everyone seems to think you can't use content from two PSN accounts on Vita without having two memory cards. I decided to try it for myself.
I'm pleased to report that it's not true.
This morning I connected to both the US and Japanese PSN Stores and downloaded content from both. I did not switch memory cards (I only have one), or reformat the memory card.
I am logged on with my main US account and all the content from both regions is playable. :)
The way I did it is still a bit of a pain, but unlike the two cards method, you only need to do mine once, and at the end everything is on one card and playable under your main account. The only limitation I've found is that I get a message telling me that I will be unable to earn trophies on the Japanese content when I start playing it from my US account.
For those of you playing at home, here's what I did:
DISCLAIMER: You might want to backup your memory card first. You really don't need to, but if you do this wrong you could wipe your card which would suck.
Choose Settings > Format > Restore the PS Vita System
You'll be asked if you want to deactivate the system from your account first.
Yes, you do want that.
Wait. Deactivation occurs.
Select Restore again.
This time you'll be asked if you want to format your memory card during the restore process.
No, you do NOT want to do that.
The restore process completes leaving your card contents intact.
Set your language & timezone.
Choose to sign in with your Japanese (or whatever other region) PSN account.
The unskippable Vita intro movie plays. Make a sandwich.
Download Japanese content from the Store (or from your PS3) onto the Vita.
When you're done, follow this process again, from the top, but choose to sign in with your main account. Remember to Activate your Vita for Games when you're signed back in.