Storage is PS Vita memory card. It's the only one, you don't have any other way to storage anything. (4/8/16/32gb) That's the main reason they made a special SD kinda Card to avoid piracy.
While the PS Vita Game card holds 5–10% is reserved for patches, saves. So when you buy some game *technically* the game card is not full, it holds some *free* space to avoid fill memory space so fast.
Or experience too much lag or some weird problem with the console.
That's why i don't see the cartridge idea happening.
So to make it easier and avoid confusions:
PS Vita GAME CARD: Games also some free space for storage but for the game only.
PS Vita MEMORY CARD: Storage.