Currently, there are vaccines available for at least 12 different disease, including important intranasal vaccines for influenza and strangles. Intent to vaccinate depends chiefly on the risk. Look at these 3 factors. 1/ the likelihood of the horse getting the disease. The risk may vary with the locality, season, age and the management of the horse. 2/ The potential severity of the disease. What is the cost? 3/ The safety and efficacy of the vaccine.
The major vaccines are: 1/ tetanus, 2/ strangles, 3/ influenza, 4/ encephalitis and 5/ rhinopneumonitis

A good scenario for the concepts needed for some of these equine diseases is "A Case Scenario: Equine Respiratory Disease by Carrie McColgan at http://www.vet.uga.edu/vpp/ia/srp/ERD/index.html