A bit late to the party, but I'd guess it was bonded with some kind of LOCA (Liquid Optically Clear Adhesive) glue. It's also used in mobile phones to stick LCD to front glass.

If so it's possible to stick them back together, you can find both LOCA glue and LOCA dissolver (if you want to try and replace only front glass so you need to clean LCD from old glue) on ebay. There's also thin (very thin) wire that used to cut through original glue and separate screen from glass on mobile phones. Heat the screen with heat gun, lift glass a bit, get wire in and slowly heat and cut.

But luckily my vita is still OK so I don't have to test all of these theories :D