Hello, No Signal means the TV has completely lost all signal from the Roku, which can happen due to an HDMI cable that has failed (which they can do without warning), the Roku HDMI out failing or the TV's HDMI ports failing.
That said, have you tried:
1) Checking to make sure you selected the correct HDMI INPUT on the Vizio (using the INPUT button on it's remote)?
2) Tried the other HDMI ports and selecting them respectively on the Vizio?
3) Purchasing another HDMI cable?
Those are the first steps you should try, as this is the basic troubleshooting protocol anytime an HDMI signal drops and you're left with a "No Signal" message.
Beyond that, I will need you to TEST another HDMI source (such as a Blu Ray player, Cable Box, etc.) on the Vizio, OR try the Roku on another TV to isolate the root cause.
In the case you find it's the TV and none of the HDMI ports work (even with a different cable) then this rules that the Vizio set's HDMI processing on the Mainboard has failed which requires replacement of that part.
In that case, that is also something I can help you with, but I wold need the age and the full model number off of the back of the Vizio to proceed on that end.