When I first entertained the idea of buying a Vita the glaring problem of memory card prices put me off BIG time, but I eventually bought this very sweet piece of kit... my tips as follows, along with facts.
First off, don't buy the 4GB card if you have slow broadband(which I doubt most people have).
I bought Uncharted with my Vita, and this requires only 64MB for a save.
Trophies and stuff that you unlock are stored on-line, so saves *can* be deleted once you complete a game.
All your Vita games will be bought from shops, so if you want to demo a game before buying one, the biggest demo will be around 1gb, just make sure you have the space. Delete unused demos, as remember if you want to play any demos again you can just re-download them which will take about 30mins for most people.
What I'm saying is that most of your games will be Vita games, and they come on their own card.
Personally I think I will never need a bigger card for at least 12months, as once I've played a demo enough, I either buy the game or not, either way the demo gets deleted.
Finally, PSP downloads... this could cause space problems. I have my eye on Eternal Blue, which takes up 822MB and costs just £9.99. I have two options, as there really is some great PSP titles available. I can buy 2-3 at a time and store these on my 4GB, or I can be sensible and buy one and finish it or get sick of it and delete it(remember save games remain, so I can re-download the game in the future and continue where I left off) and download another.
This 4GB is all you need if you have a good Internet connection, and don't mind doing a bit of file management(delete unused demos, videos, psp games etc...).
If you're unfortunate enough to have slow broadband(is there such a thing?) then get the biggest card money can buy, for the rest of you - this is the saner option in my opinion.
Due to the availability of full PS Vita games at good prices on the PSN store, you WILL need a bigger card if you decide to download big files.
I still stand by this being big enough for the reasons I gave - just buy the games second hand to save money and space.