Vizio TVs traditionally haven't upscaled to 4K from lower than 1080 very well, but that was zero concern for me. First, they do fine with 1080 HD sources, and I have all upscaling done before the TV. Either by the disc player or AppleTV 4K, both of which do an excellent job.
I just wanted to make sure. I have the (I think) 2020 version of the 55" M Series in my living room, and it's just an impressive TV, especially for the price, which was less than $500. It doesn't handle HDR as well as the P Series I have downstairs, but for the price, it's pretty amazing.
The Vizio M Series is a good TV. A good value, and better in most ways than anything available ten years ago. Still, there are also much better, and more expensive options today, though some aren't all that expensive. Currently, I think TCL might take the prize for best bang for the buck in...