I recently picked up Project Diva ƒ for my NTSC Playstation Vita. The version I bought was ASIA. How would I buy DLC for it?
You'll need to create an Asian PSN account (I recommend Hong Kong because the store is in English, not sure about other Asian territories) and buy your DLC from there, which means you'll need point cards for the appropriate region. Just remember, though, that Vita can only have one account per memory card, so you'll basically have to start fresh on that account to use the DLC. You won't be able to carry over any progress from your other account.
For some games, you can do that. I don't think it's consistent, though. I wouldn't risk your money on it.