There are a lot of opinions based on anecdotal evidence. There is also a lot of scientific research that both confirms and debunks some of that stuff.
20 weeks POL is average for many breeds. Birds reaching that age going into autumn/winter will take longer because day length is the critical element in sexual maturity.
So, it does make a difference that we are in the dead of winter. Increasing day length induces production in both hens and roosters. Think spring, a time for growth and optimal time of year to reproduce.
Cold weather can have an effect due to stress but day length is the foremost determinant. Any kind of stress (disease, environment, predation, nutrition, changes in routine, etc..) can inhibit ovulation.
Chickens, like all earthbound animals, need a dark period. If one wants to provide heat (which isn't normally necessary) it should be from a non-light producing source.
Vinegar is an acidifier. Acidifiers help them assimilate calcium. It limits the water founts from getting slimy. Raw apple cider vinegar contains live cultures and enzymes that promote gut health.
Layer feed (average of 4% calcium) should be limited to flocks where all are actively laying. That works for a lot of people with a handful of chickens all the same age, as well as commercial layer operations where the birds are not only the same age, but also, are photo-stimulated to start laying simultaneously.
Any bird(rooster, growing and molting birds) not actively laying should not be getting such a high level of calcium.
The best approach for a flock with one or more birds not laying is to feed an all flock or grower feed and provide oyster shell free choice in a separate container for those hens that are laying to get what they need when they choose.