I'd recommend keeping the memory card. A Vita 1000 does not have any internal storage so if you ever decide to remove the hack to reinstall or whatever, you'll need a memory card to hack it again.
A Vita 2000 and a PSTV have 1Gb of internal storage that can be used to hack the device on firmware versions 3.60, 3.65 and 3.68 - these can always be hacked without a memory card.
But even if it's not necessary: selling it won't bring in much money and you can always use the card as additional storage for PSX/PSP titles to be used with Adrenaline or ROM storage for Retroarch. Basically, it's another 16Gb to be used for content.
But basically:
Got a Vita 2000 or PSTV? - You can sell it if you want (i.e. it's not needed for hacking)
Got a Vita 1000 with Enso? - You can sell it, but if the hack ever gets lost you'll need to repurchase a memory card
Got a Vita 1000 without Enso? - You can't sell it, it's required to run h-encore.