Activate and set up the render elements you’ll need for compositing. Next, activate the “VRay FRAME BUFFER”. Inside the VFB rollout, check the “SAVE SEPARATE RENDER CHANNELS” check box and click “BROWSE” to save your frames. When you click render, Max will prompt you with an alert about “saving a sequence with no output”, click “OK” to ignore. Now VRay will save out all your render elements in the specified folder. Each frame will have the element name suffixed to the original file name.