There is no standard height for a vanity mirror. The vanity mirrors currently available on the market range from 30 inches to 72 inches high, with specialty mirrors going beyond this range.
The reason that vanity mirrors are not uniform in size is that choosing a mirror depends on several factors, such as the size of the vanity and the size of the bathroom, among others. A common rule for a good-sized vanity mirror is that it should not be wider than the vanity. When considering the people who use the vanity mirror, the mirror's uppermost edge should be a few inches above the tallest person in the house.