Do dipole dipole interactions need to be added explicitly to vdW forces while doing DFT simulations for organic molecules?
(Permanent dipole)-(permanent dipole) interactions are covered by the electrostatics of DFT (and Hartree-Fock, for that matter, because no [Coulomb] electron correlation is involved). In my opinion, interactions between a permanent dipole (such as the one of HCl) and an instantaneous dipole (methane) are covered by the parametrization of dispersion-accounting methods in an averaging fashion.