If you were actually referring to or V-Ray for SketchUp , the versioning is slightly different (e.g., 5.10.03), so please specify the host software if you need the exact changelog for that specific tool.
V-Ray 5.1.3 is not a marquee version. You won’t find "Introducing V-Ray 5.1.3!" banners on the Chaos website. Instead, it is the —the update you installed, nodded approvingly at, and then forgot about because it simply worked . vray 5.1.3