Your chickens should be good down to zero, as long as you have proper ventilation and roosting bars that let them keep their feet flat so they can cover their toes with their feathers. Roosting bars that they grip expose the toes to the cold and are difficult for the hen to insulate. Here in Florida, it seldom gets below 40, so I can't give advice on vasoline and its use on wattles for frost bite. Keep a close eye for issues by examining the birds for things that just don't look right.
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Too little ventilation keeps in moisture and ammonia. This will cause respatory disease.
Riki