ux0:id.dat is the file that links a memory card to an account.
MID=[some PSN ID] DIG=[some PSN ID] DID=[some PSN ID] SVR=[FW version] AID=[Same as CMA folder id with reverse byte order] OID=[PSN name]
If the various PSN IDs do not match up to the values stored in the system settings (in the Registry), then the Vita will prompt the user to format the memory card or not use it. Even if the values of the PSN id in the system is faked, it still will not allow you to play any games if the system is not activated on PSN.
Bypassing Version Check
The Vita will always write the current system version to the SVR field in the file. This means if you have two Vitas on different FW versions, and you put the memory card from the newer FW Vita into the older FW Vita, it will prompt you to update. To bypass this, you can make the id.dat file read only.
SceIoStat stat; stat.st_mode = 0x00002004; sceIoChstatForDriver("ux0:id.dat", &stat, 1);
This will make any attempt to write to id.dat fail including attempts to update the SVR field.