Set where you want to start and adjust the frame how you want, go to a later point in the timeline of the pan/crop window, add another key frame and adjust the frame to where you want it to end. That should do it. Vegas will "move" the frame from the starting point to the ending point.
You would need to know keyframes in order to do that. On your still there would be a box looking with a point up and a point down. It's a little more difficult for me to explain it in writing. The other guys on this forum know how to explain it better. I'm more visual trainer than writing. God Bless