I often turn to the Help Documentation in Vegas Pro and wanted to clarify that one of the examples used to demonstrate compositing is correct, but only if the Parent Track is in Multiply (Mask) Compositing Mode. By default, Vegas Pro has its compositing modes set to Source Alpha, and so if you didn't know this, you might get even more confused. Vegas has extremely powerful compositing powers, but the key to using them is understanding these core concepts clearly.