These experiments investigate the photodissociation of methyl vinyl ether at 193 nm in a crossed laser-molecular beam apparatus. We observe two C-O bond fission channels, a minor channel producing CH3+CH2CHO (X (2)A(')) and the major channel yielding CH3+CH2CHO (A (2)A(')). Some of the neutral A state vinoxy product undergoes secondary dissociation to produce ketene+H. These experiments on the photodissociation of methyl vinyl ether, which produce nascent vinoxy and methyl radicals cleanly in a one-to-one ratio, serve two purposes. First, using the measured photofragment velocities and product branching we calibrate the relative sensitivity of mass spectrometric detection to the methyl and vinoxy polyatomic radical products at the m/e=15 daughter ion, taking into account the loss of neutral vinoxy to ketene+H formation. This relative mass spectrometric sensitivity calibration factor is determined to be 0.116 +/-0.022. Knowledge of this factor allows us to extract the product branching ratio between the two major competing primary product channels from the O(P-3)+ethylene reaction, a branching ratio that has been much disputed in the literature. Our results give a CH3+HCO/H+CH2CHO product branching ratio of R=0.61 +/- .11, corresponding to 38% (+/-5%) branching to the CH3+HCO channel. Second, we use the result that the channel producing A-state vinoxy dominates over the formation of ground-state vinoxy to test propensity rules being developed to help predict what product channels may be suppressed by electronically nonadiabatic effects in chemical reactions. These propensity rules discriminate between channels that are "electronically facile" and "electronically difficult/prohibitive." We find that a qualitative analysis of the changes in electronic configuration along the reaction coordinates for the photodissociation of methyl vinyl ether correctly predicts the dominant channel to be the production of excited state (A) vinoxy, the electronically facile channel.