Can someone explain exactly how the whole region free/lack of region free works? Can I play Japanese games on the same card as my US games without getting DLC for the Japanese ones? If I want DLC, does that mean I need another memory card? It's confusing...
You can play imported physical cards on your main US account. Simple use the cardrige.
However, online pass and DLC for imported games do not work with your US account. Keep that in mind when importing your games.
To use a imported online pass, get DLC, demos, etc from a different region, you have to created accounts for those regions.
To switch between regions you do not need a second memory card, just do the following:
1) connect your vita to PC or PS3.
2) in content manager, choose "backup" to save all your US account stuff to PC/PS3.
3) in system options -> Playstation Network -> System Deactivate, choose to deactivate your device in both options.
4) in system options -> Playstation Network, choose "reset your device". It will make your Vita goes the the initial state, everything is cleaned, and next time you boot the device it will prompt you to add a PSN account. Unplug vita from PC/PS3.
5) use that other region PSN to log in Vita, you will be able to get/use everything in that account. NOTE: trophies earned in this account will not count towards your main account.
6) when you are ready to go back to your main account, just connect the vita to the PC/PS3 where you made the backup and on content manager, select "restore". No need set your main PSN account stuff, a restore put the vita in the state it was when the backup was created. Note: if you want to keep some stuff in your other region account, make a backup of that PSN account first.
Note: it seems that demos can be shared between the accounts. When using your other region account, connect vita to PC/PS3 and on content manager just select "transfer from Vita to PC/PS3" and select the individual demo you want to share. After you restore your main account, go back to content manager and select "transfer from PC/PS3 to vita" and select the individual demo file to transfer it back to vita.
But this process is quite a pain since a backup/restore can take from 20 minutes to 1 hour depending in the ammount of things you have, and you cannot do anything else with the vita while in the process.