Interest in alternative sources to synthetic N fertilizer for switchgrass (Panicum virgatum) production continues to grow. Nitrogen fixing legumes interseeded into switchgrass may be one alternative. Common vetch (Vicia sativa) is a N-fixing legume that occurs naturally throughout the Southeast as well as other parts of the U.S. It has less hard seed than hairy vetch, making it less of a weed problem and common may fix N at similar rates to hairy vetch (Vicia villosa). The objectives of this study were to: a) determine and compare nitrogen fixation rates of common  and hairy vetch when seeded into established stands of 'Alamo' cv. switchgrass and b) estimate stand densities needed to fix the recommended N rate (67 kg ha^-1) for switchgrass biomass production.  In this study, N-fixation rates via the N-difference method were determined to be 59.3 and 43.3 kg N ha^-1 for common and hairy vetch, respectively when seeded at 6.7 kg ha^-1.   At these rates, neither common nor hairy vetch significantly affected switchgrass yields. Based on the N fixation rates and vetch plant masses determined in this study, we estimate that minimum seeding rates of 7.6 and 10.4 kg PLS ha^-1 of common and hairy vetch will be required to obtain plant stands needed to fix the current recommended rate of N for switchgrass biomass production.