There soon should be a Vegas plug-in from Borris Red that supports much more advanced masking via the layer approach whereby you can in effect 'draw' on your video and create masks of complex objects that once made can travel with the object you're masking. It won't be cheap, probably costing more than Vegas. Which is pricy for a plug-in.

You can mimick it to some extend using something like Photoshop. The problem would come from trying to track the mask if the object you're masking has a lot of motion. If you're just trying to mask a static object like a wall or something else behind foreground objects then the other methods work with the warning any color in the objects you don't want to mask bleed into what you do the results can be disappointing and require a good deal of trial and error and frequently trade off.

I forget which thread, but Dennis from SoFo, briefy explained a way more suited for masking parts of a video so some objects are black and white while others remain color by using composting. I did it a simpler way as explained in one of my tutorials, but my method is more of a change color A to color B, with out trying to remove all color except for one or two objects. Again, you can do that more or less with multiple instances of the color corrector secondary filter.