We show that by preparing ethylene (C2H4) in selected rotational levels of the C-H stretching v(9)(b(2u)) = 1 or v(11)(b(1u)) = 1 state by infrared (IR) laser excitation prior to vacuum ultraviolet (VUV) laser photoionization, greatly simplified, rotationally resolved pulsed field ionization-photoelectron spectra can be obtained, leading to reliable rotational assignments of the vibrational bands for C2H4+((XB3u)-B-2; v(4)(+)(torsional) = 1 or v(11)(+) = 1) <-- C2H4(X(1)A(g); v(11) = 1) and C2H4+((XB3u)-B-2; O0+) <-- C2H4(X(1)A(g); v(9) = 1). The observation of these Franck-Condon (FC) forbidden vibrational bands is indicative of a strong state-mixing mechanism, in which Rydberg states converging to the FC forbidden C2H4+((XB3u)-B-2; 0(0+) and v(4)(+) = 1) levels are effectively coupled via intermediate states to Rydberg levels converging to FC allowed C2H4+((XB3u)-B-2; v(9)(+) = 1 and v(11)(+) = 1) states, respectively. This conclusion is supported by strong autoionizing features observed in the IR-VUV photionization efficiency spectra.