Yep thanks for the prompt reply. I d/led Vegas 6.0 tryout and could not see how you did it in that either. Can you explain how it is done ? It dosn't seem as easy as changing the audio one.
I'm guessing that you inserted the opacity line in Vegas 6 by dragging down the top of the event. This is a static line that can't be changed. Go to insert / video envelopes / track composite level. Then you'll have an envelope line that behaves the same way as the audio envelope.