This process is possible, though slightly awkward. Given that you require a V-Ray license, you would wish to maximise its use by installing it with Rhino. However, if you are setting up a render node, or farm, you may wish to only extend your V-Ray for Rhino rendering capacity. For this, it's necessary to have V-Ray and V-Ray Distributed Render Spawner installed on your node.
The workaround is as follows:
The V-Ray for Rhino installer will look for Rhino System folder. To get round this, create a folder named Rhino6 and a folder System within it. When using the V-Ray installer, point it to this new directory:
C:\Program Files\Rhino6\System
This should enable V-Ray to be installed as a node without Rhino. Bear in mind that the licensing process remains the same and it will require access to a server where the V-Ray license service is running.
When using distributed rendering, V-Ray requires access to the V-Ray license server. Once V-Ray is installed on the render server, it must be told where to look for the V-Ray license server.
This can be set using a V-Ray utility (Start menu > Programs > Chaos Group > V-Ray for Rhino > Licensing > Administration > Change V-Ray client license settings).
One V-Ray for Rhino license allows up to 10 machines to be used for distributed rendering.
Links for further information: V-Ray for Rhino | V-Ray Help