While classic theories utilize the comparison between cómp[ɛ]nsate going to comp[ə]nsátion and cond[ɛ́]nse going to cond[ɛ]nsátion to argue that stressed vowels are immune to reduction in multiple affixations (e.g., SPE), this paper presents a corpus-based case study that looks into this quantitative interaction between vowel reduction and stress shift during English -ion nominalization and offers discoveries that go against the classic claim. After analyzing 1,047 verb-noun target pairs extracted from the CELEX2 dictionary corpus, this study claims that vowel reduction only partially depends on its stress-bearing feature and that the suffix type, the stress shift pattern, vowel tenseness, and crucially some lexically specific constraints also predict vowel reduction. This finding is further supported by an OT analysis and a statistical model. As a quantitative study that relies on an exhaustive list of English samples to derive theoretical analysis, this research not only provides new insights into this long-lasting debate but also aims to highlight the significance of incorporating large data samples for a complete understanding of phonological phenomena.