I don’t think this has Dolby Atmos. The sound bar shown has no top speaker grilles. The Dolby Atmos version is the M512A-H6.
Vizio’s own product page says that the M51a-H6 is a Dolby Atmos system.
This does not have “physical” Dolby atmos. Meaning there are no upward firing speakers. However, it processes programs(movies/shows) that offers Dolby Atmos. I’ve had 2 of this model. One was returned and replaced with another. Eventually returned the replacement also; due to several malfunctions (read the reviews/problems online; especially Reddit). Yep, I do not recommend.
How is this 5.1?
The soundbar has three speaker areas: center and left and right. It’s a pretty narrow soundstage but you can tell the difference. The subwoofer is wireless (plugs into an outlet) and the surround speakers plug into the subwoofer with nice long cords.
Atmos is baloney, they use some sort of virtualization processing but it’s not real.
eARC works well.
So 3 distinctive speakers are housed in the casing or just a few components and some are shared components?
I had a 7.1 traditional system since doing some remodeling I have my back and surrounds up but my fronts aren’t. I put in a marble wall where my tv now is mounted on. if I can get a soundbar with 3 speakers that I can hookup to my receiver it’ll save me trouble of drilling more holes in the marble. do they even make something like this? 
You need a “3 channel passive sound bar”
I think it has to do with HDMI-CEC I bought one a few years ago and it was a pain in the ass to deal with definitely wouldn’t recommend anything Vizio. But my OLED and the Atmos have been for the most part operating correctly now.