Diagnosis
This is almost always caused by an external device waking the TV up via the HDMI-CEC feature, which Vizio calls 'System Standby Control'. Your cable box, game console, or streaming stick is sending a 'power on' signal.
How to Fix It
Go into the TV's Menu > System > System Standby Control. Turn it to 'Off'. This disables the HDMI-CEC function. Next, go to Menu > System > Timers and make sure 'Sleep Timer' and 'Auto Power Off' are also off. Finally, unplug any devices from the HDMI ports, wait a minute, and plug them back in to reset the handshake.
If That Didn't Work
Check for a firmware update. Go to Menu > Admin & Privacy > Check for Updates. Older software had bugs that could cause random power events. An update often resolves it.
Heads Up
If turning off CEC doesn't stop it, the issue might be with the TV's main board. A failing capacitor can cause erratic power behavior. If it's under warranty, that's your next call.