Creating a shallow DOF in post is never truly convincing when done in video; tools like DeFocus are more for 3D models and separating subjects from backgrounds.
That said, you can use Bezier masks or mattes to cut out a main subject and apply very light blur to background content, or the Radiance plugin for Vegas has a very nice Defocus feature when combined with masking.
Your best best of course, is to accomplish this in-glass. Either by backing way off, using zoom to accomplish the shallow DOF on closeups, or using a tool such as the RedRock Micro 35mm adapter to extend length and give you that DOF. With the new flip feature in Vegas, it’s quite handy.