Welcome to Ja,
My Name is John,
The problem you are describing could be a failed HDMI controller on main board, but i need to know have you tried to see if the YCB connection puts out any signal, i know it is annoying to run 5(3 video and 2 audio) but unlike what the salesmen tell you HDMI is a very small improvement over YCB 3% to be exact and that is only when running 1080p otherwise they are the same on any cable length smaller then 20ft, as YCB starts losing quality as the length gets longer and HDMI can run 100s of feet with no lose.
If it is a failed HDMI controller you would need to replace the main board, which at a minimum is over $200 plus VIZIO parts are so unreliable you may need to replace the new one because it will not work (swear to god it happens all the time vizio sends dead board, because they do not take any care in shipping and send them in a plastic bag and the static kills the new boards)
Vizio has a very high failure rate of there main boards especially the HDMI jacks i have done 4 of them this month all HDMI controllers melted after a power loss and otherwise the set worked. In most cases i recommend you have the board replaced professionally since you then don't have to out of pocket 2 boards as the company issues a refund after the board is sent back and it can take up to 60 days in some cases.
I have to say if you don't mind YCB connection (component) it is the best route cheap and fast, gets the job done, and no lose of picture quality.
Let me know if this helps or you have any questions,
If you do have a failed main board i need the serial number and any remaining model number info ie VX42l HDTV10A if that is on the end, the serial number is XXXXX because the same model can be built by 8 different companies so it could have 4 different main boards and the only way to find the right one is the serial number, VIZIO make a great picture but since they have to cut money somewhere it comes down to support, hardly any support on the aftersale on VIZIO products it can be hard but we can get the info if we need to...