Blur out an irregular shaped section of the frame using Sony Vegas Studio
Unfortunately you are asking too much from Sony Vegas my friend! In order to get a true effect you'd have to do something called "rotoscoping" which means you go into each frame and mask out the background.

You can do this in vegas by going into the pan/crop window on the video event. Look for "mask" and then you can key frame out the background with the pen tool there.

There is no way to automate this so you'll have to key frame quite a bit. Then once you have just the person visible you duplicate the track beneath it and uncheck the mask on the duplicate. You then can add lense blur or gaussian blur to create the desired effect.

Unless the clip is super short I wouldn't recommend doing it this way though. It takes a lot of time even for skilled maskers.