Maybe it can be fixed. At the material object properties there is a little “plugin” flag. If this flag is disabled than it can’t be enabled again and there is no way to connect the object with the Vray material again. Also it dosn’t help to assign the material from the Vray material editor again, the connection is destroyed for “ever”. Could it be fixed?
try this: vray menu --> tools --> material utility tools --> enable all vray materials 
Great, that is it. Thank you.
If you want to individually re-enable a V-Ray material in Rhino 6, simply click on its name in the Material Properties panel and choose “Use Plug-in: V-Ray for Rhino” from the dropdown list.
Kind regards,
Peter
Found it.
Thank you very much,
Micha