Perhaps I should have said, "It will just guarantee fogging if your eyepieces aren't heated. Not allowing heat from your face, breath, and warm moist eye to fog the eyepiece usually, in my experience, requires allowing ventilation between the eyepiece and your eye and that is the opposite of a hood that extends forward on either side of the eyepiece. I very rarely ever observe at a place that is warm enough at night to allow the use of a black cloth over the head or an observer's hood. With either, one breath would cause instant fogging of a cold eyepiece. I don't have a problem with fogging because I don't breathe on the eyepieces and I have sufficient eye relief that my eye doesn't cause fogging.

I have used an occasional eyepiece that required complete blockage of peripheral light to prevent visible reflections off the eye lens (one example: 9mm 120 degree ES). A hood or black cloth would work great on an eyepiece like that (easier than cupping one's hands on either side), but I would expect the eyepiece to fog up. You can always use heater cords on the eyepieces, but I prefer not to do that. I'm fortunate enough to observe in arid, non-humid, environments.

An observer's vest to hold eyepieces sounds just great, but I have about 12+ pounds of eyepieces and PowerMate in my case, and I just couldn't see carrying that in pockets, even if the stitching were strong enough. But, then again, I don't use a ladder with my scope. A vest for eyepieces would be quite practical in that environment.