Q: On my main floor that has plywood floor, do I need to put down black 15 lb. construction paper before installing my 3/4″ hardwood flooring?
A: It certainly is not and could not be classified as a vapour barrier. Nor should you need one, unless you have a swimming pool in your basement. Such paper does slow moisture transfer which may be important if you have a very damp area beneath the floor. Many people simply use a wax coated paper now which will assist in the boards moving into place much more easily.
You should make sure the moisture content of the existing plywood is not more than about 4% higher than the finished floor and that the finished floor is in the 7-9% range.