If you're using a V-Ray version earlier than 3.1, please note that the BRDF section of the materials is set to Microfaset GTR (GGX). If your version is older than 3.1, the BRDF field will be empty. Choose Blinn, Phong, or Ward – whichever is more preferable for you.For Corona render, it's recommended to use version 1.5 or higher, as the glossiness of materials is adjusted for PBR.