Next week, I will buy PS Vita when my mom come back from the overseas... I can't buy the DLC because I forget my ID and password... :'( So, can I get the DLC from my friend??
Congrats on your mom returning! And you can just go online and retrieve your password (on your vita there might be a "forgot my password" button when opening the market). I wouldn't recommend using a friends account due to the complexity and problems caused by it.
I already know my ID and password when I listen to "Rin-chan Now!" song.. :3 So, I can't use my friend's DLC to mine??
I don't think you can transfer DLC from someone else's account, I'm pretty sure you need to get it yourself because it's tied to your PSN account. Don't quote me on it because I have no idea if that's how it works exactly. Sony is a little funny that way so it wouldn't surprise me.