  We show that measuring the $\cos 2\phi $ angular dependence in unpolarized
Drell-Yan processes with $\pi^-$ beams colliding on proton and deuteron targets
can determine the ratio of the Boer-Mulders functions for $d$ and $u$ quarks
inside the proton $h_1^{\perp,d}/h_1^{\perp,u}$, which is still lack of
theoretical constraint. The comparison of the $\cos 2 \phi$ asymmetries
measured in unpolarized $\pi^- p$ and $\pi^- D$ Drell-Yan processes, which are
accessible at CERN by the COMPASS collaboration, can help to discriminate
whether $h_1^\perp$ effects or QCD vacuum effects are preferred by data.
