I have Buttercups in Massachusetts. And they got frostbite on the very tips of their combs no matter if I put Vaseline on them or not. And that was in a completely enclosed barn with a flat-panel coop heater near their roost. I had some that were in Chicken Jail who did not get Vaseline on account of they kept roosting in the rafters where I couldn't catch them, and some that were in the regular coop who did get Vaseline. No difference. Both areas were in the barn.
I wouldn't get them again. Not so much the comb thing, that wasn't such a big deal, but they are super-flighty and hate being handled. And both my Buttercup roos are dumb as a box of rocks and aren't at all nice to the ladies.
They are good at laying through the winter, I'll say that for them. The good points are, they are really reliable layers and they can forage and evade predators like you would not believe. Two of mine escaped and wandered the neighborhood for about a month in the dead of winter, and I finally caught them in my neighbor's tree. As healthy as could be, just rather thirsty. We've got coyotes and fisher cats here, plus a couple feet of snow, yet they were OK with just a touch of frostbite on the comb and wattles. I just hate their personalities.