The effects of interactions between the genotype and environmental conditions are expressed in the phenotype. Comparing the performances of genotypes under the same range of environmental conditions allows for relative measurements to be made of the different levels of plasticity among those genotypes. The objective of this research was to evaluate the different responses of two wine grape cultivars, native Aglianico and international Cabernet Sauvignon, under different pedoclimatic conditions in terms of the functional traits that govern grapevine bud fruitfulness, vegetative growth, and yield development. The study was conducted over two consecutive seasons (2020 and 2021), in six commercial Vitis vinifera L. vineyards, located in three distinct viticultural regions of central-southern Italy (Molise, Campania, and Sicily). In each experimental vineyard, the bud fruitfulness, number of leaves, total leaf area per vine, midday vine water status, and fruit yield were measured. The obtained results showed that bud fruitfulness was higher for Aglianico than for Cabernet Sauvignon in each experimental site, while the variability of plant responses between the vineyards was lower for Cabernet Sauvignon cultivar compared to those for Aglianico. The genetic × environmental interactions were expressed predominantly during berry ripening stages, while plasticity was generally greater in Aglianico than in Cabernet Sauvignon.