Evaluating graft quality in grapevine grafted plants is challenging because structural differences at the graft union are mostly internal. Conventional nursery assessments rely on external inspection, which provides limited information on vascular reconnection. This study applied two complementary approaches to characterise graft union quality: histological observations of callus and vascular development, and hydraulic conductance measurements. Trials were conducted in 2020 and 2021 with Tempranillo grafted onto 110 Richter. Five bench grafting methods were tested: omega with completely aligned cambium (OM-CA), omega with partially aligned cambium (OM-PA), V-shape (V), full cleft (FC), and hand-made whip and tongue (WTh). Histological analysis based on phenotyping of graft union vascular development were done at two time points, after the callusing phase and after field rooting, and hydraulic conductance was measured in one-year-old potted plants using a gravimetric flow–pressure method. Omega grafts, despite shorter contact surfaces, generally produced higher callus and a greater proportion of plants with complete vascular connection. OM-CA reached the most advanced vascular developmental categories, whereas V and WTh remained in intermediate stages and FC showed variable results. Hydraulic conductance did not differ among methods, although OM-CA displayed the highest and most uniform values. These results indicated that grafting efficiency depended more on cambial alignment and tissue differentiation than on the length of cambium surface. Combining scalable histological and hydraulic measurements offered a practical framework to assess graft quality in nursery trials and complemented advanced imaging approaches.