what are the most common vaccinations that you would reccomend? Are there any uncommon ones that are only needed in certain circumstances? How often should the horse be vaccinated?

There are quite a few vaccines that can be given. I found this link and it seems like a good one. I think it depends on location, laws and other factors what you vaccinate against.

http://www.horseadvice.com/horse/messages/3/5104.html

I hope this answers your questions.