Hairy vetch (Vicia villosa Roth) is an over-wintering annual legume valued for its ability to provide N through symbiotic N fixation (SNF). Although SNF is a high priority trait for farmers, evaluating SNF in a breeding program is a challenging endeavor. Our objective was to evaluate how SNF varies within and among hairy vetch genotypes, and how SNF covaries with traits directly targeted by breeders (i.e., vigor and flowering time). We evaluated SNF in two hairy vetch breeding nurseries (2017 and 2019) based on the proportion of N derived from SNF (Ndfa) measured using the δ15N natural abundance technique. We also considered plant size (vigor) and flowering stage. In 2017 we sampled plants on the same day, at varying developmental stages. In 2019 we sampled plants at a common developmental stage (early flowering) on different days. The 2017 data could not be confidently interpreted because of confounding effects between developmental stage and genotype. The 2019 data collection controlled for developmental stage effects and showed a positive relationship between vigor and Ndfa that appeared to be driven by flowering time. In other words, genotypes with delayed anthesis produced greater biomass and newly fixed N. All three traits showed discernable variation among genotypes and likely reflect cold late winter/early spring conditions that limited growth and N-fixation of early-flowering plants. Taken together, our study shows that developmental stage and community composition are important considerations for evaluating SNF in annual legume breeding programs.