This has been a problem since at least Vegas 16. I've tried newer Vegas versions all the way up to the latest Vegas 21 and the problem remains. The Starburst effect renders differently depending on if you have "GPU Acceleration Of Video Processing" turned on in the video settings. Specifically the main difference is if the option "Center Burst" in the Starburst effect is turned up. You get a residual image of whatever you applied the starburst effect on casting to the upper left. Looks like this:



With "Center Burst" all the way down, you get this:



If I max "Center Burst" again and change to "Points" to 2 only and leave the 45 degree angle from bottom left to top right, you can really see this effect and its issue:



The distance of the 'ghost image' moves further away the more you increase the "Radius" option.

Now if I restart Vegas and disable "GPU Acceleration Of Video Processing", this is what the effect looks like with "Center Burst" maxed:



This to me looks far more correct of how it's supposed to look. However there's plugins and effects that rely on GPU Acceleration (such as Picture In Picture), so disabling GPU Accel isn't a good solution. Starburst with GPU Accel off also has other strange artifacting, such as some strange cut off lines when increasing the Radius too far:



Or even weirder dupe images at the edges of the burst if you're at a specific radius like 20 and with Center Burst maxed:



I have tested actually rendering out to see if it was just a preview thing, but it does actually render the way it shows here in the preview window in Vegas, both with GPU Accel on and off, which seems to change how the effect renders. I really like this effect (when it works) and would love to see this get fixed at some point.