Vaccination is rarely 100% protective. Vaccination does not deal with PI animals. They are too highly infectious and will still be able to spread infection potentially creating yet more PI animals. If you vaccinate a PI calf, it will have no effect on the amount of virus that is shed by that calf resulting in a constant viral challenge on the farm. In some cases and for various reasons some animals do not respond to the vaccine effectively - if this does occur then the cow or heifer may be at risk of producing a PI calf in subsequent gestations.
Vaccination plays an important role in eliminating BVD from the farm but should be carried out in conjunction with appropriate on-farm surveillance and, when they are present, identification and removal of PI animals.