What happens in your body if you are exposed to a virus for which you have been vaccinated?

If the microbe is one you've been vaccinated against, and if enough time has passed since the vaccination for full immunity to have been developed by your immune system, then your immune system will be able to recognize and destroy it before it can replicate and cause illness. See the related question below for more information about why you could still get infected even after having received a vaccination, although that is the exception to the rule and usually it does not happen that way.