I also have a Vizio soundbar, but luckily don't have any issues with my setup. Great inexpensive soundbars, btw.
I know the soundbar has rubber pads on it's bottom, but get some tiny self adhesive rubber feet, not flat ones, but round ones like this,
https://www.amazon.com/Self-adhesive-Clear-Rubber-Feet-Bumpons/dp/B001JAW454
Use 4 on the bottom of the soundbar. This will help minimize contact to the cabinet. Also, pull the soundbar as far forward to the front of the cabinet as possible. I would set bass and treble flat and see if the other adjustments, TVOL and TSHD, affect it any. Then try adjusting bass and treble.
I know the soundbar has rubber pads on it's bottom, but get some tiny self adhesive rubber feet, not flat ones, but round ones like this,
https://www.amazon.com/Self-adhesive-Clear-Rubber-Feet-Bumpons/dp/B001JAW454
Use 4 on the bottom of the soundbar. This will help minimize contact to the cabinet. Also, pull the soundbar as far forward to the front of the cabinet as possible. I would set bass and treble flat and see if the other adjustments, TVOL and TSHD, affect it any. Then try adjusting bass and treble.