no longer able to get HDR or dolby vision for gaming on vizio TV

Dolby vision is not 4k it is a software enhancement for the HDR aspect so not having it is not a major issue, but if you like to have all the green checks you need to look for high end tv's that support Dolby Vision like LG Oleds and Vizio Oleds

I would hold off just a little while longer for a 4K TV. There are competing standards for HDR, with the two main ones being HDR-10 and Dolby Vision. The difference between them being the colour depth and peak brightness. HDR-10 is the standard
chosen for UHD Blu-rays. Dolby Vision is the more advanced of the two but requires more processing hardware. Dolby Vision supports HDR-10 by default, but not the other way round.

But know that HDR and Dolby Vision are two different, incompatible, color schemes for UHD TV sets. Most support HDR but some support Dolby Vision. There are a couple of websites I check for this kind of tech talk but I don't think we csn post links to
outside sites here. Most UHD tvs support HDR 10 but I've heard Dolby Vision is better. Xbox One does not support Dolby Vision, just HDR 10.

Current list of TVs that support Dolby Vision with Xbox is available at https://www.dolby.com/us/en/categories/tv/dolby-vision-xbox-compatible.pdf

Currently Vizio is not in the list, and the list will be updated as and when Dolby adds a support for them for Xbox.

On your console display settings, is HDR and Dolby Vision enabled?

Most people that get flickers on the screen (both playing or only on the dashboard) with Dolby Vision on is because both HDR and Dolby Vision are enabled at the same time.

Just try to turn HDR off and see if it solves your problem. If it does, unfortunately we will not be able to do much about it. It's a bug reported by some people and the only option would be use HDR or Dolby Vision, not both at the same time