this happend yesterday, the PS button blinks and fades (its like when you hibernate the vita, and the ps button blinks and fades after couple of seconds) then after about 10 mins, it blinks again, though it never shows any notification or any status. I turned off all notification and still... the PS button blinks again.
When the Vita is in standby mode it signs in to PSN every 10 minutes or so to check for messages, activity updates, Near content, and similar items. The PS button does that while the Vita is online in standby mode.
The notification settings have nothing to do with it. The PS Vita always wakes up from standby every 10 minutes or so. If you don't want it to do that, then you have to power it off by holding the power button down for a few seconds, and then tapping the Power Off button on the touch screen.