Corner pinning is what you refer to, and unfortunately it has not yet been implemented into Vegas Pro's motion tracking setup. I think Hitfilm can? I know Mocha can if you're willing to pay for it... Or you can wait for the Vegas team to add it in a future update, which I'd imagine they would (though I can't say for sure, I've heard nothing about it yet).

This is what I want. Would like to feature walls and windows and such within a clip replaced with video and/or text, manipulated in 3D to match on-screen geometry, tracking to corners. I don't understand why the auto-track can't just be inserted into 3D masking features, since it's just the same thing applied to a set of coordinate transforms that are already in the 3D engine and baked into every video card on the planet since the 90's. You're just using the on-screen coordinates, gathered from the motion tracker, to inform the same 3D transforms you would otherwise create manually, that's all. Every 3D game does this as a matter of course with on-screen vertices, and the math is already in the rotations and sizing you do manually within Vegas's 3D engine. ME WANT NOW, this is fresh-out-of-college coder stuff that should be in there already.