The reaction of acetaldehyde cations with ethene has been studied as a function of collision energy and acetaldehyde vibrational state. REMPI through different vibrational levels of the B electronic state is used to produce CH3CHO+ with controlled excitation in different vibrational modes. Reactions are studied in a guided ion beam instrument, including measurements of product ion recoil velocity distributions. In addition, we calculated the structures and energetics of 13 different complexes that potentially could serve as intermediates to reaction. Three reactions are observed. Hydrogen atom transfer (HT) dominates at low collision energies and is suppressed by collision energy and, to a lesser extent, vibration. The HT reaction is clearly direct at high collision energies but appears to be mediated by a reactant-like precursor complex at low energies. The most energetically favorable product channel corresponds to elimination of CH3 from an intermediate complex. Nonetheless, this channel accounts for only similar to0.5% of the total product signal. The cross section for endoergic charge transfer (CT) is strongly enhanced by collision energy in the threshold region. Over a wide range of collision and vibrational energy, CH3CHO+ vibrational excitation enhances CT, but only 18% as much as for the equivalent amount of collision energy. This effect is interpreted in terms of competition between the CT and other product channels. The expected proton-transfer channel is not observed, an absence also attributed to competition.