Looks like the Vegas Flicker Control fx is broken VP20 current build. Setting: Timelapse default (ie: "Histogram Smoothing").
Completely garbles the picture (see screenshot) when used in 32-bit video levels mode (with Vegas-created ProRes 422 footage).
In 8-bit video levels, Flicker Control induces a color shift (same mode: Histogram Smoothing) because it seems to be acting on a per-channel basis.
I personally don't rely on Vegas Flicker control.... I use Digital Anarchy Flicker Free.
OK, let me make an impassioned plea for why this plugin is useful. Deflicker, Neat Video, and FlickerFree are all paid plugins that address flicker--but they are hard to use and come at terrible render time costs (ie, 10X).
For situations in which you're dealing with complex flicker, yes, that's what you have to turn to.
But for the very common situation in which you're working with footage shot on a camera using auto exposure, and that produces visible brightness jumps every time the camera adjusts its exposure on a static scene (like GoPro), the Vegas fx does a good job eliminating that, and VP Flicker Control is very light on render times.
It's a very useful plugin--or it would be if it worked. Note that it is listed as 32-bit compatible, so it appears to be broken in at least two dimensions.