A simplified "five vacant hybrid orbital" model is proposed for the boron-vacancy complex, and the defect wave function are calculated using an empirical tight binding Green function method under extended defect potential approximation. The calculated hyperfine interaction constants of the nearest neighbors of the vacancy and the probability distribution of defect wave function localized at atomic distants from the vacancy, are in good agreement with the electron-nuclear double-resonance experiment. The deep energy obtained is comparable with the value measured by deep-level transient-spectroscopy. The theoretical results provide a strong support to the conclusion that boron atom is located at the lattice position next to the vacancy and the defect is a B-V+ charge state.