The Backyard Poultry Magazine has a good article on vaccinating poultry in the Spring 2019 Special Subscriber Issue. It starts on page 44 and is written by Michelle Miller, who has a doctorate in Immunology and experience as a poultry vaccine research and development scientist. She also keeps a backyard flock at her home in North Carolina. I can't think of anyone with better qualifications to deal with the subject. While it does not go into the risk of exposure from showing poultry, it does deal with questions like, "What if I already have birds that are not vaccinated, and are introducing vaccinated birds to the flock?" and "Will vaccines make my birds sick?" It's a short article, but a good read.