Maize production is highly vulnerable to rising vapor pressure deficit (VPD), yet its agricultural-scale impacts remain insufficiently characterized due to the lack of integrated assessments linking spatiotemporal VPD dynamics with crop-level water productivity (WP), particularly the relative contributions of yield and evapotranspiration (ET) under varying atmospheric drought conditions. Using the Coupled Model Intercomparison Project Phase 6 (CMIP6) data, we analyzed the spatiotemporal evolution of VPD across five maize ecoregions in the Yellow River Basin (YRB) under three Shared Socioeconomic Pathway (SSP) scenarios (baseline: 1985-2014; future: 2021-2100) and quantified its effects on maize WP. Mean VPD across the YRB maize ecoregions is projected to increase by 21% from 2021 to 2100 (vs. 1985-2014), peaking in the 2080 s under SSP585. Eastern ecoregions (II > I) experience greater VPD escalation than western counterparts (V > III > IV), with post-2030s VPD growth accelerating to 0.036 kPa/yr under SSP585, while SSP126 stabilizes. Projected VPD increases may hinder WP sustainability in regions II-V across all climate scenarios. ET primarily governs WP under mild atmospheric drought (VPD <= 4 kPa), whereas yield constraints dominate under severe drought. Thus, a VPD-dependent shift in the WP demands a process-based optimization of irrigation and yield management under varying atmospheric drought conditions and offers actionable insights for climate-resilient agricultural strategies in (semi-)arid maize-growing regions.