This plugin is common to all versions of Vegas and is v1.0 in the 'about' section of the plugin.
Basically it goes loopy when it's applied, even when the sliders are at default 'do nothing' settings.
It causes the upper ranges to go out of bounds, so you get a big, overbright smeary section on things like skies, or bright surfaces. No amount of adjust the available sliders can bring those things back into range either. They're just permanently mangled.
A sort of workaround is use Curves, just before it, to bring the whole range down quite a lot (so bright white is more of a muddy grey) then it can be applied to...push the brightness back up where it was before without it going out of range.
See the before and after when it should be doing nothing...yet.
http://i854.photobucket.com/albums/ab106/pickaname2/HSL%20Off.jpg
http://i854.photobucket.com/albums/ab106/pickaname2/HSL%20On.jpg