Make sure you don't turn the box off at the plug or switch on the back of the box, also make sure the box is also set to fast start or active start and don't actually sign out of the apps, just press the TV button to exit and go back to live TV.
If you still need to sign in again to your apps then it sounds like you have a faulty box, if that's the case then you need to call Virgin on 150 from a Virgin phone, mobile or landline, or 0345 454 1111 from any other phone, choose the option to report a fault, then ignore all the other options offered and you should then be placed into a queue to speak to someone.
The other option is to wait for one of the forum team to pick this up which might be 2 to 3 day's.