A card like this would probably cost as much as the Vita itself. But if Sony ever did release one of these, I'd buy it in a heartbeat.
I currently own four Vita memory cards: my current 64GB memory card is almost completely full; my second 64GB memory card I use mostly for storing PSP games; I have a 32GB memory card used for storing movies and TV shows; and my old 8GB memory card that I'm thinking of reusing for a US account.
If not for the fact that you need to power off the console before swapping memory cards, these multiple cards wouldn't be that big of a deal. It'd be so much easier to have everything on one card. Ironically, despite the Vita's awesome back catalog of digital PSP games and ability to use a second thumbstick in PSP games that desperately need it, it's more cost effective and convenient to just buy a $2 micro SD to Memory Stick Duo adapter and just play them on a PlayStation Portable using an inexpensive 128GB or 256GB microSD.
As much as I prefer physical games, I can see the benefit of keeping digital copies of them. No need to swap out game cards. But they can take up several hundreds of MB to several GB in size, especially with DLC and patches. And with the cards maxing out at 64GB all that free space is quickly gone. Ironically, despite getting digital Vita games for cheap during sales I ultimately found myself sourcing out physical copies just to free up some extra space on my main Vita memory card.
Sony just doesn't seem to realize that the Vita's ultimate deal breaker was its memory cards. Normally price is not an issue for me, but what they are charging for these memory cards is ridiculous. Go onto ebay or Amazon and you'll find the 64GB cards selling for about $120 or $160 or even $200; I can walk into a JB Hi-Fi and purchase a 256GB micro SD card for about that price! And I can buy a 64GB micro SD for less than $60.
Sony desperately needs to start releasing Vita memory cards in the 128GB and 256GB range and charge them at what they are currently charging for the 64GB card. Or better yet, why not just release an adapter that will allow us to plug in one or two micro SD cards into the game card slot, kind of like giving the Vita a second hard drive. I'm sure that's not outside the realm of possibility. If the hacking community can get microSD cards to work in the Vita, I'm sure Sony can release a firmware update and an adapter to allow us to do it officially.
Bottom line, with the physical Vita game cards being discontinued outside of Japan next year, Sony desperately needs to do something about the limited storage if they expect us to go all digital from 2019 onwards. Even if Sony charged an official SD card adapter for about the price of the 4GB memory card, I'm sure it would be substantially more cost effective and conveniently than where we are now.