Hello doctors, can vaccines be rendered useless like antibiotics if they are overused. e.g if a new strain of smallpox was?

Not Exactly: it is not over-vaccination that causes vaccine resistance so much as mutation of the organism, whether a virus or bacteria. Antibiotic resistance occurs when resistant organisms pass genetic information to non-resistant organisms. This is more likely to happen after someone has been exposed to an antibiotic that was not appropriate for the condition, the wrong antibiotic, or the wrong duration.