As far as I know Vegas has no mechanism to project a shadow onto a surface in 3D.
You can however create the illusion of one. Ideal way is a 3D application but I doubt you want to go down that road. Simpler way is to draw a suitable black trapezoid shaped object in PS or several of them and composite them in using Vegas. You could even use generated media with a bezier mask to create the shadow objects. All this is doable with a fair amount of frustrating / rewarding work so long as the surface the shadow falls onto is a smooth plane like a table top. If it has contours or very rough texture and you want something photorealistic then you really do need a 3D applications and a lot of time.

Having said that some of the free 3D apps are capabale of creating shadows and you can extract just the shadow and composite it in Vegas.

Bob.