Diagnosis
The TV isn't detecting the active input signal. On Vizios, this is almost always the TV's CEC/device control feature fighting with your connected gadget.
How to Fix It
Disable CEC on the TV first.
1. Press Menu on the remote, go to System, then CEC.
2. Turn 'System Audio Control' and 'One-Touch Play' to OFF.
3. Unplug both the TV and the Roku from power for 60 seconds. Plug TV back in, then the Roku. Turn them on and check the HDMI port again.
If That Didn't Work
Try a different, known-good HDMI cable. If the problem persists on all ports, perform a power cycle: unplug the TV, hold the physical power button on the back of the TV for 30 seconds, then plug it back in.
Heads Up
If you use the TV remote to control the Roku, turning CEC off will break that. You'll need to use the Roku remote directly.