My understanding is that saving your game does not require a card. I can't remember, but I'm inclined to say you can save on the card like a regular cartridge game. I think there was supposed to be built in memory for saving, but I think that was scrapped to hack a bit at the price or something. But I'm 99.999999999999% sure that you don't NEED an extra card to save on. The card is more for digital games.
edit-AHA, got this off the Wiki...assuming it's true "PSVita memory cards will be available in size from 4 GB to 32 GB, with 4 GB versions available at launch, with 5–10% of the writeable space will be reserved for save data, patches, etc"