How To Make an Advanced Loading Bar in Sony Vegas
If the spark footage has an alpha channel, as is likely, then make sure, it is activated - right-click on the event, launch Properties, and check that Alpha Channel in the Media tab is set to Straight.

Now, animate the Spark's position in its Pan-Crop window to track the leading edge of the bar.



Offhand, I don't know if you parent a track, is the parent's Pan/Zoom inherited by the child tracks. Try it out. For Track Motion, it is, but that reduces the resolution of the footage pre-compositing to fit the project dimensions, so I avoid it.