It did seem to help my hens with large combs. It acts like a moisturizer to them just like it does to us
My rooster has a rose comb so he was a little better off, but I went ahead and did him as well and there was definitely nothing negative that happened.
I used vaseline on my chickens when their combs got a little chapped last winter. Two had a touch of frostbite, but I went ahead and treated all of them preventatively.
Combs always seem to look and feel dryer in the winter (at least to me). If you want to add a thin layer of Vaseline and rub...