lol wth that saller 

U can play any region game. But vita bound to 1 PSN. So chose wisely.
If u buy R3 game u can't download DLC from PSN R1.

Yes u can't use DLC...u can't co op with other region for certain game.

PSvita not like Nintendo. U can play at any region. My Vita was HK set, I register under US PSN and subscribed PS plus and play all the game I download from PSN.

To expand a little further...
There are sometimes Japanese titles (R2) distributed among few R3 PSN... if ya getting the R2 physical copy, I believe you are able to download the DLC (if they offer) in R3 PSN that distributes the same title...

Online play is really depending on the game iteself, some games allow players to play with other players of different PSN but some just don't...

Also to be safe, just go with Physical copy, don't ever bother with digital, Sony has the worst for the owner of digital game copies...
At least Nintendo comes clear with the region lock, Sony has been going back on their words quite often or kept changing statements to confuse you... I had everything Sony game systems except PS3 and beginning from this VITA, I began to hating Sony more than loving it, I can't find myself supporting this abysmal corporation anymore... Players outside of Japan are treated like shits by Sony, think and choose the PSN wisely if you really have to stick with PS Vita...