Unfortunately, we do see examples of bacteria and viruses evolving vaccine resistance. For instance, vaccine resistant strains of polio and pertussis have recently been identified.
Yet these seem like the exception rather than the rule. One thing that makes it harder for pathogens to evolve resistance is that vaccines usually generate antibodies to multiple antigens, all of which the pathogen would have to alter.
Another reason vaccines tend not to drive vaccine resistance is that the diseases for which we vaccinate are typically those that are not antigenically variable. (Haemophilus influenza is a notable exception; there we use a conjugate vaccine). This may have something to do with how we view vaccine efficacy. For example, we don't say that the influenza virus evolves vaccine resistance--yet we need a new vaccination each year due to antigenic shift.
If you are interested in learning more, Angela McLean has a nice article looking into this question.