(Document not fully finished)
For rendering object ID in VRay, we just need to add the attribute of it and then change the ID in the attribute editor. Then we can add an object ID render element for rendering them out.
Select hat
Adding VRay Attribute
Object ID Setup
Render result
And then if we can render the Multi Matte Channel using our object ID. If you want to render the multi matte layer, we just need set an color mapping between the object ID and RBD channel, then we can add multi matte render element in render settings for rendering them out.
Create and select Multi Matte Channel
Setup ID and RGB Channel in Attribute Editor (need to select the multi matte channel in Render Settings)
Multi Matte Channel Render Result