We report on the successful implementation of a high-resolution vacuum ultraviolet (VUV) laser pulsed field ionization-photoion (PFI-PI) detection method for the study of unimolecular dissociation of quantum-state- or energy-selected molecular ions. As a test case, we have determined the 0 K appearance energy (AE₀) for the formation of methylium, CH₃⁺, from methane, CH₄, as AE₀(CH₃⁺/CH₄) = 14.32271 ± 0.00013 eV. This value has a significantly smaller error limit, but is otherwise consistent with previous laboratory and/or synchrotron-based studies of this dissociative photoionization onset. Furthermore, the sum of the VUV laser PFI-PI spectra obtained for the parent CH₄⁺ ion and the fragment CH₃⁺ ions of methane is found to agree with the earlier VUV pulsed field ionization-photoelectron (VUV-PFI-PE) spectrum of methane, providing unambiguous validation of the previous interpretation that the sharp VUV-PFI-PE step observed at the AE₀(CH₃⁺/CH₄) threshold ensues because of higher PFI detection efficiency for fragment CH₃⁺ than for parent CH₄⁺. This, in turn, is a consequence of the underlying high-n Rydberg dissociation mechanism for the dissociative photoionization of CH₄, which was proposed in previous synchrotron-based VUV-PFI-PE and VUV-PFI-PEPICO studies of CH₄. The present highly accurate 0 K dissociative ionization threshold for CH₄ can be utilized to derive accurate values for the bond dissociation energies of methane and methane cation. For methane, the straightforward application of sequential thermochemistry via the positive ion cycle leads to some ambiguity because of two competing VUV-PFI-PE literature values for the ionization energy of methyl radical. The ambiguity is successfully resolved by applying the Active Thermochemical Tables (ATcT) approach, resulting in D₀(H–CH₃) = 432.463 ± 0.027 kJ mol⁻¹ and D₀(H–CH₃⁺) = 164.701 ± 0.038 kJ mol⁻¹.