The discrete variational method, cluster model and a fully-three dimensional numerical relaxation technique are used to study the atomic configuration, positron annihilation characteristics and electronic structure of the V-N-vacancy complex in the dilute Fe-V-N alloy. Optimum location of nitrogen and vanadium in the V-N-vacancy complex is determined, according to the result of the positron annihilation lifetime experiment and minimizing the total energy. The bond characteristics and charge transfer in the V-N-vacancy complex are analyzed.