I think its used to connect to the displays ot stores, but Im not sure. I havent seen the vita display at the store since I got mine in FEB so I cant remember if it attaches at the top or bottom. It seems to have a metal screw hole to screw in the thing that holds it to the display maybe.
It would be cool if they could make a videoout or to hook it to a ps3 for some stuff.
Also vita doesnt need a trigger adapter because it can use the touchscreens to act as L2/3 and R2/3 for ps2/3 games.