As far as I know there is no way to see a 100% view using Vegas' preview window. You can see non-scaled images if you have a secondary monitor (and uncheck "scale images to fit"), but if you want to see a 1-to-1 view in the preview window, there is no direct way do that.
I was thinking maybe I could use the clip pan/crop to crop in as a hack. The idea would be to crop in so that the frame size matches the preview window size (which is helpfully shown below the transport controls below the preview). But I'm not sure if that's actually a 1-to-1 view.
If I crop way, way in to an image, say to where the frame width is only a handful of pixels wide, Vegas doesn't show me individual pixels -- instead it shows some kind of average. I have no good explanation for this behavior. If I crop to, say, 18x18 and set project properties to an 18x18 frame size, Vegas still shows a blur/average instead of actual pixels.
So maybe there's no way to see a true 100% zoom in the preview window.