Where are Vita saves stored?

I want to borrow some Vita games off a friend, I was wondering if I might need to worry about overwriting his own saves.

With PS3/PSP games I know saves are stored on the device itself (since the game media is read only), but since Vita has its games on memory cards, are the saves on the game media itself? (like GB/DS games)

Unfortunately, the answer is that it depends on the specific game.

From the PS Vita manual:

Two types of games are stored on PS Vita cards. One type saves data on the PS Vita card and the other type saves data on the memory card for the PS Vita system. To determine which is the case for a specific game, see the software manual for the game.

If you want details for a specific game, a lot of publishers now put their manuals online.