Also, there is a smart Vegas plug-in (freeware) which does this. It's called "Route Animation Filter" and is available for download via -> this website.
Look an easy way to do this in Vegas is after you have made your path layer with a transparency.
Create a small mask square 4 corners on the track then simply move the mask square along the track and make keys progressively
Then return to each key and move the bottom nodes of the mask out, takes a few seconds.
This is with step one and two together so you can get the sense of it.
LRH (LDC), I've been using Power Point for this sort of thing and just exporting the animated slide(s) as an MP4 video (1280 x 720p) which you can then edit on the timeline as you wish, works great for me. The animation facilities in PP are goood enough for most of my maps/diagrams etc and it's very easy to use. Horses for courses maybe?
If you have your map image you select = paint as an object, then you paint the line freehand and select mask option in your graphics layer= same as deselect the background and save. This is will give you the line transparency layer same size as map cause you don’t want to resize graphics in Vegas.
Some great ideas here! Thanks everyone. Byron, where will you post the tutorial? Looking forward to it seeing it. Also going to experiment with the PP idea suggested by Baldbiker. Cheers, LRH