This is usually the case when Vray can’t find the .dll from the shader, because the environment variable is not set up correctly.
Check by using
getenv "VRAY_FOR_MAYA2016_PLUGINS_x64"
as mel code. It should point to the folder with the dll.
You could also try copying the .dll into vrays bin folder.
If that does not work either take a look at vrays output when the plugin is loaded. It should print something like this:
[2016/Jun/22|11:44:54] V-Ray: V-Ray core version is 3.25.01
[2016/Jun/22|11:44:54] V-Ray: Loading plugins from “C:/Program Files/Autodesk/Maya2016/vray/vrayplugins/vray_.dll"
[2016/Jun/22|11:44:57] V-Ray: 74 plugin(s) loaded successfully
[B][2016/Jun/22|11:44:57] V-Ray: Loading plugins from "C:/Users/administrator/Documents/maya/2016/modules/Houdini_Ocean_Toolkit/vray/vrayplugins/vray_.dll” [/B]
[2016/Jun/22|11:44:57] V-Ray: 1 plugin(s) loaded successfully
If it finds the .dll but has a problem loading it, there will be an error message there.
Cheers,
Nico