if a point in cylindrical cordinates is A=(r1,\theta1,z1) and another point is
B=(r2,\theta2,z2)
(if ar,a\theta,az are unit vectors)
then the position vector of A is = r1<ar>+\theta1<a\theta>+z1<az>
and the position vector of B is =
r2<ar>+\theta2<a\theta>+z2<az>
so vector BA=...