The ability to mask "On-Screen" with the VEGAS Bezier mask has been amazing for me. Like many have suggested, one thing that's missing is our ability to zoom in the subject in order to mask him out. Using Smart Zoom or Pan Crop tends to make it worse, since the mask does not stay on the originally assigned region.
I think what would be a nice middle ground if the preview engine upgrade isn't in this year's road map is the ability to "Zoom" inside the Bezier Masking tool.
See how "Smart Zoom" behaves with the Location attribute? Throw it in Bezier Masking so that we are able to zoom and navigate around the image/video without deterring the original location of the mask we started.