High-entropy carbides (HECs) are novel advanced materials composed of covalently bonded multicomponent transition metals and carbon. The transition metal atoms of all the precursor components are randomly distributed at the cation sublattice. Recently, a series of HECs with a non-stoichiometric compound (NSC) as one of the precursors have been synthesized. However, the effect of vacancies on the formation of HECs has not been addressed well. In this paper, NbC0.5 was selected as the NSC to study the interfacial diffusion behavior of NbC0.5 and other covalent transition metal compounds (MCs: M represents one of V, Ti, Ta, and W). The presence of vacancies resulted in a carbon concentration gradient, reduced the diffusion activation energy of metal atoms in NbC0.5-MC and increased the diffusion driving force. In addition, the diffusion process of WC in NbC0.5 was analyzed. Phase transformation of WC to W2C was found during diffusion. The experiment proves that HECs may be easily prepared with the participation of the NSCs, and this work provides the theoretical basis for the alloying of covalent transition metal compounds.