Short answer is there are more than two layers.
A perfect key would involved only two layers: the top being the one you’re keying and the bottom your background.
In the case of this if you simply do a luminance key or chroma key out the white the whites of the eyes will be keyed as you pointed out, not the desired effect.
So you add another layer on top, this one is a sort of reverse garbage matte. You use Vegas 5’s bezier mask to coarsely mask out the key, leaving the edges of the model to be keyed out with the keyer. I typically have the middle layer (the one to be keyed) muted so I can see the results of my mask.
Now I use combustion and after effects for this sort of thing but I BELIEVE with Vegas’ parenting you can parent the two layers and add a colour correct to the top layer and it will ripple through to the siblings (as in Combustion).
Is that what you’re going for?
– Rick