All vaccines have primary and secondary failure rates. Primary failure occurs if you get vaccinated but you do not develop antibodies (This is rather crude description of immunological memory but let just stick with it). Secondary one occurs when you get vaccinated, you develop antibodies but once you contract the disease you still get it (e.g. you immunological memory does not kick it). Both of these are well known effects and are subject of research to develop more effective vaccines.
So even in population where 100% people get vaccinated there is insignificant percentage of those who are still susceptible to given disease.