Thats strange. Maybe I don’t fully understand the situation.
https://www.sonycreativesoftware.com/webhelp/vegaspro/12/ENU/Content/CompositingandMasks.htm
I used to be able to do this in Vegas years ago. I think last version I was doing this was v4. of course been using more recent versions since.
But I have just been trying this agin with no luck in v 12. Using the settings in the docs above.
Not working at all how it is described.
Should be able to go:
Parent (Mask)
Child
Parent
And the last one should show through.
“In this example, the third track is also a parent, so it is displayed behind the composite of tracks one and two.”