I have been playing with this.
I watched the (rather fast) marketing videos on the Vegas website and sort of worked it out.
You have to apply a Bezier mask first, to the clip to which the text is to be anchored. I am not sure why that is, other that Vegas decided to add the Tracking function to that fx. It doesn't really matter what config is used in that Bezier fx, it seems.
Then one has to use Tools / Scripting / Add text to motion track, to get the text. One cannot do it with e.g. Insert Text.
The text then appears on a newly generated track, at the top of the project.
What I found is that when this is done, one can delete the Bezier fx from the clip, and one can move the text to the same track on which all the other text objects are. Otherwise, you could end up with many tracks, of annotating e.g. a flying movie.
Presumably there is something about that Bezier function which tracks some pattern in the image, so the fx needs to be positioned within the image where there is some stuff that has clear edges. Does this make sense?