There may be multiple settings for this within your TV's menus. Did you try contacting Vizio support to ask for some additional help? You can reach them here: https://support.vizio.com/s/contact-us?language=en_US
You might also try going into Settings>System>Advanced system settings>Control by mobile apps and disabling network access. If someone were using the Roku mobile app while connected to the same wireless network as your device, this setting would prevent the app from communicating with your Roku device.
Keep me posted!
Thanks,
Tanner
I have the same problem but send everything through a Yamaha AV Receiver. Haven't figured this out yet, but not liking this feature of Roku. This doesn't happen with my AppleTV 4K or the old Fire Stick I have. I got the Roku just for Youtube in 4k (AppleTv doesn't support 4k) but it may not be worth it.
Edit: Forgot my solution. I also have new Vizio. This is partially the fault of the vizio but I switch the AV receiver to another input and then when I turnoff the Receiver the screen changes to No Signal for 10 minutes and then it will power down. If I forget to switch from Roku to another input then the TV stays on all night. I'm working on the Vizio and Yamaha settings but haven't found anything yet that will help other than turning off CEC and that causes other problems...