In order to enhance the thermoelectric properties of Si-based bulk materials, the lattice thermal conductivity (κ<inf>lat</inf>) should be reduced with little degradation of the electrical properties. Our group had previously demonstrated that the phosphorus (P)-rich nanoscale precipitates formed naturally in heavily P-doped bulk Si scatter phonons more effectively than carriers, resulting in a high figure of merit (ZT) [A. Yusufu et al., Nanoscale 6, 13921 (2014)]. Here, we successfully prepared heavily P-doped bulk Si containing metallic VSi<inf>2</inf>nanoparticles through the ball milling of Si, V, and P and subsequent spark plasma sintering. The VSi<inf>2</inf>nanoparticles did not affect the electrical properties of Si significantly but greatly decreased the κ<inf>lat</inf>, leading to a marked increase in ZT. The maximum ZT value, which was 0.4, was obtained at 1073 K, which is two times higher than that for Si.