This model TV has a history of main board issues that will give exactly what’s going on in your case where the standby lamp lights when the set is plugged in then fades away and the set is dead after that, unfortunately that board isn’t cheap. The 12 volt supply is actually 19 volts, Vizio has the legend wrong. I’ve fixed several sets with this issue and its always been the main, the power board in this set seldom gives issues. To double check I have a D55u-D1 on my bench that had backlight issues and is fixed now and to be sure I was thinking of the correct model I tested the 12 volt supply and it measures 19.2 volts on a working TV. Also the 5 volt supply is not a standby supply, so will be around 0 with the set off. As the above poster said you need 3.3 on/off voltage to turn the power board on, this set is totally different than most sets that have a 5 volt standby that is always there when the set is plugged in

You have a bad EEPROM in the main board, its what sends the start boot to the CPU, its a common failure, and a $10 part but a little tricky to replace unless you are real good at soldering, its only a 8 pin chip but very small, a replacement main board will also fix the issue but that board isn’t cheap