I'm having the same issue. Bought the Roku for the same reason as well to have all the components in one hand held remote. This is also a Vizio TV but not a Smart TV this was pre-smart anything. I've tried both HDMI ports and nothing, all I get is a "no signal" flash on the television screen.
Whatever it is it's Vizio related not Roku. I have the same exact stick on my Samsung 8xxx series Tv and it worked fine immediately.
My router/Modem is about 6'-8' away. No walls to block any signal.