The crystal structure, electronic structure, and thermoelectric properties of a half-Heusler VFeSb (HH-VFeSb) compound are investigated. The HH-VFeSb compound is successfully synthesized by arc-melting, annealing, and spark plasma sintering processes. The crystal structure of the HH-VFeSb compound is refined by Rietveld analysis of the synchrotron-orbital-radiated X-ray diffraction pattern. The refinement results reveal that the HH-VFeSb compound has a deficient HH-VFeSb crystal structure rather than an ideal HH-VFeSb structure. The 4a and 4c sites are approximately 10% deficient in V and Fe, respectively. A small amount of Fe atoms occupy 4d sites, which are typically vacant in ideal HH-VFeSb. An antisite defect between V and Fe atoms likely exists in HH-VFeSb. Based on the revealed crystal structure, the electronic structure of deficient HH-VFeSb is calculated using density functional theory, which reveals the origin of the n-type HH-VFeSb compound from the interstitial Fe atoms at 4d sites. From the comprehensive crystal and electronic structural analyses, we conclude that the V- and Fe-deficient HH-VFeSb crystal structure is the dominant reason for the electron conductivity of the HH-VFeSb compound. A maximum zT of ca. 0.35 at 550 K is obtained, which is one of the highest zT for the HH-VFeSb compound.