A high-level computational study using CCSD, CCSD(T), and G2(+) levels of theory has shown that unactivated vinyl substrates such as vinyl chloride would afford gas phase, single-step halide exchange by a pure in-plane sigma -approach of the nucleophile to the backside of the C-CI sigma bond. Geometry optimization by CCSD/6-31+G* and CCSD(T)/6-31+G* confirms the earlier findings of Glukhovtsev, Press, and Radom that the S(N)2 reaction of Cl- with unactivated vinyl chloride in the gas phase occurs by a sigma attack. Complexation of vinyl chloride with Na+ does not alter this in-plane a preference. However, moderately activated dihaloethylenes such as 1-chloro-1-fluoroethylene undergo gas-phase S(N)2 aback by the accepted pi -route where the nucleophile approaches perpendicular to the plane of the C=C. In the latter case a single-step pi pathway is preferred for the Cl- + H2C=CFCl reaction. This is the first definitive example at a high level of theory where a single-step pi nucleophilic vinylic substitution is preferred over a multistep mechanism in the gas phase. The activation barriers for these gas-phase single-step sigma- and pi -processes involving both naked anions and Na+ complexes are, however, prohibitively high. Solvation and the presence of a counterion must play a dominant role in nucleophilic vinylic substitution reactions that proceed so readily in the condensed phase. In solution, nucleophilic vinylic substitution reactions involving electron-withdrawing groups on the carbon-carbon double bond (e.g., -CN, -CHO, and -NO2) would almost certainly proceed via a free discrete carbanionic intermediate in accord with experiment.