Create traveling matte of just the actor walking on layer 2, cutting out all the plane background. Keyframe the matte to follow the actor.
Then apply the actor layer over the background layer.
Once you start to rotoscope elements, you will see why shooting a plate shot and green screen layer will save time.
There is other software like Blender and Mocha that can do this type of stuff better. In that software, you edit only the shot you need, then reimport the composited footage back into vegas for editing.