Vaseline smeared thickly on the skin does offer protection from hyperthermia in swimming events if you are not using a wetsuit. I've never heard of this being used for naked hiking. I prefer to wear a T-shirt on my back to stop the heavy back causing soreness. People sleeping naked will learn quickly what they need and don't need — sleeping with a naked partner is a sure bet to success: something warm and cuddly to snuggle up to however so you wish to! A big doona is fine for me and a day of solid physical activity so I am tired when night comes and can sleep solidly.
I know it may not be totally in the buff, but my legs get chilly a lot, so I like to wear a sarong as a sort of long slit skirt. I am warm, but it's not heavy, so I don't fell like I am wearing anything.