I think this is the same for Vegas 4 and 5
In the video preview pane, the floppy disc icon on the far right is used to save a still. TIP- The save function only saves he still as you view it, so select Best (Full) preview mode for the best picture.
To view frame by frame, zoom the timeline as far as you can and then move the cursor along the timeline. If you disable Snapping ( F8 ) you can get closer still.
Hope this helps