Did you download the 64 bit version of Vray for Rhino from the Chaos Group site? This will be the version that will show up in Rhino 5 x64. If you already downloaded this and installed it with Rhino closed, it may just not be loaded. If so, check Options>Rhino Options>Plugins in Rhino 5 x64 to see if Vray is present in the list of plugins installed. If it is there, you probably just need to check the check box to load it.
Does that help?