This paper introduces the social network procedure of cohesive blocking (Moody & White 2003) as a strategy for fine-grained quantitative network analysis in sociolinguistics. In Raleigh, North Carolina, the Southern Vowel Shift is reversing, due in part to large-scale migration from outside the South since the mid-20th century. Acoustic analysis of the five front vowels from a 140-speaker subset of the conversational Raleigh corpus reveals steady change across apparent time. The community's network structure is considered via a bipartite, or two-mode, network of schools and individuals. Cohesive blocking generates a network hierarchy in which individuals are "nested" at different levels. Nestedness is then tested as a predictor of linguistic variation in linear mixed effects models, which reveal significant nestedness effects for three of the five vowels, net of age, sex, and occupation. Speakers with higher nestedness lead the retreat from the Southern Vowel Shift.