The 19‐electron VCoSb compounds are actually composites of an off‐stoichiometric half‐Heusler phase and impurities. Here the compositional adjustment is systematically studied in V1−xCoSb to obtain single‐phase V0.955CoSb. Hall measurements suggest that such a V vacancy, as well as Ti doping, can optimize the carrier concentration, which decreases from ≈11.3 × 1021 cm−3 for VCoSb to ≈6.3 × 1021 cm−3 for V0.755Ti0.2CoSb. Low sound velocity contributes to the intrinsically low lattice thermal conductivity for VCoSb‐based materials. The high Ti‐dopant content results in enhanced point‐defect scattering, which further decreases the lattice thermal conductivity. Finally, the optimized n‐type V0.855Ti0.1CoSb is found to reach a peak ZT of ≈0.7 at 973 K. The work demonstrates that the VCoSb‐based half‐Heuslers are promising thermoelectric materials.