The Douro Demarcated Region in Northeast Portugal, particularly the Douro Superior sub-region, is marked by hot, dry summers that lead to significant water shortages across the soil–vine–atmosphere continuum. Traditionally, vineyards in this area have relied solely on rainfall. However, deficit irrigation has gained recognition as an effective strategy to stabilize or improve grape yield and quality while reducing the risks associated with climate variability. To investigate this approach, a study was carried out in a commercial vineyard using Aragonez (syn. Tempranillo), a widely planted native grape variety. The vines were exposed to two levels of deficit irrigation, along with a non-irrigated control, which reflects the prevailing local practice. The objective was to examine the interactions between weather conditions, physiological responses, yield, and must quality, with the aim of informing improved water management strategies. The findings indicate that deficit irrigation can effectively reduce the adverse effects of severe water scarcity during the maturation period. Specifically, irrigation at 40% of crop evapotranspiration significantly reduced water stress and enhanced physiological performance, particularly in leaf gas exchange and vegetative balance. These results suggest that even relatively low levels of irrigation can help prevent excessive water deficits that might otherwise result in yield losses and unbalanced grape ripening.