cg.earlyworm looks at using the V-Ray Scene Access Python API to modify a scene after you have already hit render. You are able to make modifications to the V-Ray Render scene file in a text editor before you actually send it to render, and this as cg.earlyworm notes can be a great thing of you want to change shader attributes, texture information on thousands of objects, or even loading extra geometry at render time.
Check out the two posts for Using V-Ray Scene Access to Modify a Scene Before Render Time here. With a relevant example of using the V-Ray scene Access Process here.
cg.earlyworm notes: One of the lesser known features of VRay is it’s ability to access information about the scene and modify it after the render button has been pressed and before it is rendered. This ability to access the VRay scene and modify it allows you the ability to create some custom solutions to problems which might not be doable inside the 3d application itself. It can also be used to workaround bugs in VRay – but only as a temporary measure to get around bugs when a deadline is fast approaching.