I have a steel roof attached to 2 x 4 perlins attached to 2 x 10 roof rafters (cathedral ceiling). You can see the underside of the steel roof from inside the room. My question is should I install R-38 faced insulation between the rafters faced side towards the room and not concern myself with any additional vapor barrier against the underside of the steel roof. It's a camp I'm renovating in the Northeast.

Mike

Right, a vapor barrier protects the rafters and purlins (or any wood past the insulation).
I think you should still have air channels leading to vents no matter what kind of roof you have, to allow for some cooling of all the wood that will now be cooking away in the summer heat, and to allow any little moisture that does get through a way to escape.
I don't recall seeing vents (ridge or otherwise) on metal roofs around New England, but maybe they all have gable windows or gable vents, or I just missed seeing any of they other types.

Yes, always install the insulation with the vapor barrier to the warm side. Never install more than one vapor barrier.

My main question would be why are you insulating? If it's a Summer camp only, in New England, I don't see the point. Is it too hot in the summer? Are you converting it for Winter use? Above advice is correct for VB, however, don't expect a big temperature difference because the cavity insulation strategy is not nearly as effective as it could be. I would explore using rigid foam below the rafters.

Building Science Corp roof venting details