Most likely you have a bad led or
Leds in the panel. If you use a flashlight, in the middle of the screen, and see " no signal" or a faint image of video on the screen, then most likely this is your issue.
Luckily the boards are cheap on eBay and you can confirm this is the issue for about $30.
You will need to swap out the power supply and LED driver board.
If you swap those out and have the same issue you have bad leds.
The leds are only available if you purchase the lcd panel assembly. In that case it will be cheaper to buy a new TV.
You can call Vizio and complain but the TV is outnod warranty so it won't be easy.
Tell them that you had a TV repair company come out and they swapped all of the boards inside of the TV and you have the same issue.
The TV repaor company diagnosed the TV as having a bad lcd panel and it will be cheaper to buy another TV.
Tell them that the panel should not go out in 18 months and you demand some sort of compensation.
Good Luck!