VRay Render Mask Property Overview
VRayRenderMask property is designed for use only when Render mask mode in VRay Options/Image sampler (AA) tab is set to Render Mask Property. With this property you can mark only objects from the scene that you want to be re-rendered.
In order to apply the VRayRenderMask property, you need to select the desired object(s) and from Render menu to choose Get>Property> VRay Render Mask.
VRay Render Mask Property Parameters
Enabled - when this option is checked and Render mask mode is set to Render Mask Property, V-Ray will re-render the objects