Either do the highlights in Photoshop or use the yellow highlight.png file in this demo project that I created. Combinations of Pan/Crop, Track Motion and the track level slider will get you where you want to go.

John, thanks for the tip.  I tried this process but the client did not like the black text no longer black.

I think there's a solution but I'm running this morning.

John got me to thinking.  I used the "subtract" compositing mode and got the desired effect with one significant difference.  The color of the highlight is also "subtracted" from the underlaying image background.  But the result is not the inverse color as you would expect because the background screenshot is not white.

More experiments.....

Assuming the text is black on a white background as in your initial example, then

1. add the text on track 1
2. Create a solid color generator on track two
3. Put the video all of this overlays (or if it doesn't overly any video, than create a solid color generator the color you want for your background) on track three
4. Set the Composite mode for Track one to Multiply (Mask). This drops the white background from Track one
5. Adjust the Pan/Crop on the highlight event as so:
   1. Turn off Lock Aspect Ratio
   2. Turn off Size About Center
   3. Under the Source heading, set Stretch to fill frame to No (critical step that enables you to truly crop instead of zoom)
   4. Adjust the Pan/Crop box so that it crops the highlight down to exactly what you want to highlight

As has been indicated already, this is kind of clumsy to do in VEGAS, but it works and the highlight is behind the text, so it doesn't affect the text color. And if you don't want the highlight to be opaque, just drop the opacity of the highlight color. After you do the first one, you can just copy the highlight event and adjust the cropping so that it is under the text you want to highlight. If you have multiple lines that need highlighting, then you may need multiple highlight events spread across several tracks all treated as track 2 in my example. For example, to acheive the results you show in your example, you're going to need five highlight tracks.