Found this idea elsewhere.
Seeing as how Vegas doesn't dither from 32bit FP to 8 bit you should be able to add your own. Add a track of 50% grey and apply noise FX. Adjust compositing level down. I'd guess you want just enough noise to create a noise amplitude of +/- 1 bits. You could probably set this on the scopes. Solo the track and adjust until the noise is just visible at the bottom of the scopes. Compositing mode should be Add I think.
Wish I could be more specific but any banding problems I normally get to see of from my lowly 6 bit LCDs so I've no way of seeing what I'm really doing.
Bob.