The local dimming of the Vizio Quantum is great. The TV has 192 zones and is able to effectively boost highlights. In a side-by-side comparison with the Z9D the P Series Quantum is worse though, as rapidly changing small areas of the backlight produce more noticeable blooming.

When compared to the Samsung Q9FN, some people may prefer the local dimming performance of either TV. We can see in this side-by-side comparison that the Q9FN (top left) loses details in particularly challenging scenes, whereas the P Series Quantum (top right) is better at keeping these small highlights. This does result in deeper blacks on the Q9FN, but at the expense of details. The Vizio P Series 2018 (bottom left) also retains details but with a raised black level due to the fewer number of zones. The LG C8 (bottom right) shows the ideal performance with perfect blacks.

If you enjoy local dimming and don't mind some variation in screen brightness or blooming, then set local dimming to 'Medium'. This does increase the overall brightness of the image though, and can result in distracting blooming around bright objects. If you prefer a less aggressive local dimming implementation or find the brightness too high after decreasing the 'Backlight' then set it to 'Low', or disable local dimming completely if you don't like the changes in brightness.

Update 01/02/2019: The name for Vizio's local dimming feature has been renamed from Xtreme Black Engine Pro to Active Full Array.

Update 07/16/2019: Text updated to clarify settings.