Hey Daniel,
Yes, I understand your question. The answer is that there is currently no plugin or expression inside of Vegas Pro to automate the simulation of shallow DOF between layers, even though there is an underlying 2.5D layer manipulation engine.

However, you can use bezier masks to crop the same image on several layers and give each layer subtle motion, to suggest depth [the oldest motion graphics trick in the book :) ] There is also currently no automatic rotoscoping tool if you wish to perform this operation on a moving object within a video event, however, Vegas Pro's keyframe control is excellent for this purpose if you have time and patience to crop by hand.