  The production of $W^{\pm}$ and $Z^0$ bosons in collisions of longitudinally
polarized protons allows to access various combinations of the polarized quark
distributions from the study of double and single spin asymmetries. We compute
the ${\cal O}(\alpha_s)$ corrections to these asymmetries as function of the
vector boson rapidity $y$ in the hadron-hadron centre-of-mass frame. Detailed
studies of the numerical impact of the next-to-leading order corrections are
presented. The theoretical uncertainties of an extraction of polarized quark
distributions from future vector boson production data are investigated in
detail.
