Hmmm.
Well lets get this strait.
A vector Is XYZ. To get a specific axis. do Vector:z() Vector:x(), y() etc...
Now if you are finding the drop hmmm.
The drop of the bullet would not change the X or Y only the Z.
So find a formula that works with the drop of the bullet. Z = Vector:z()+(Whatever the formula here)
Just to spark some ideas.