Can multiple players share one PS Vita?
Is it possible for two or more gamers to use a same PS Vita? Can two or more PSN accounts be used in one PS Vita? Or even using only on PSN account, is it possible to have multiple save games (none of the games that I have give you the option to manage save games)?
Could I switch PSN accounts/save games using a different memory stick?
Yes, having a memory card for each user is the solution. It will allow each user to have a separate PSN account and separate save games on each users personal memory card.
You will however be required to do a full factory reset each time you swap the cards.
In my experience so far each game has had no options to manage save files so multiple players can have simultaneous games under one account / memory card.
But it is worth noting that you can manage content such as save data using a PC or your PS3 via the content manager app in the vita. This will allow you to transfer save files to another vita/memory card if required and back up you saves.