B12 and vitamin K would not be something that I would use for respiratory illness. Something better would be B complex tablets, which contain all B vitamins. Tablets can be crushed into food or a little water. Dosage would be 1/4 tablet daily or a couple of days a week.
VetRx is an herbal remedy containing oil of rosemary, oregano, camphor, and pine, and works about as well as Vicks salve. What symptoms have you seen? Common ones are bubbles in eyes, sneezing, nasal drainage, wheezing, and sometimes facial swelling. There are a handful of different respiratory diseases that includes infectious bronchitis, mycoplasma gallisepticum (MG,) coryza, ILT, and others caused by mold. The Tylan or tylosin, will treat symptoms of MG.
Had you added any new chickens to your flock before you saw an illness? Depending on symptoms, certain antibiotics may help if it is a bacterial infection such as MG or coryza. The viruses, such as IB, have to run their course over several weeks to a month. Sorry that you are dealing with this illness. If you lose another bird, keep the body cold, but not frozen, inside a cooler. Then contact your state vet here, where they can do a necdopsy and tell you exactly what disease you are dealing with:
https://www.metzerfarms.com/PoultryLabs.cfm
