Some people swear by it, others don't use it at all. "It just collects dirt and causes oily feathers!".
Some things I could recommend, be sure the coop is not holding humidity. Ventilation is needed (Ventilation should be coming from above perches, not at roost level)
humidity causes frostbite in alot of cases. Putting in a heat lamp in the coop will help as well. We dub our birds to prevent frostbite. Frostbite can cause infections, infertility,
and possibly death. We cannot absolutely PREVENT frostbite with outdoor birds. Any temperatures below freezing - birds could be at risk of frostbite. I personally, and this is
my own opinion - do not believe in the vaseline method.
Ukiah