Investigations on CVD diamond deposition on steels for high performance tools development persisted over years. Literature makes out the need for an intermediate layer to improve adhesion and film quality. This work studied Thermo Diffused Vanadium Carbide (TDVC) coating deposited on AISI O1 steel by thermo reactive diffusion (TRD). Thick TDVC layers formed by varying the thermo diffusion time from 1 h to 12 h. Hot Filament Chemical Vapor Deposition reactor (HFCVD) growth conditions variation made possible diverse diamond films growth, from ultrananocrystalline to microcrystalline. X-ray diffraction, scanning electron microscopy with a field emission gun (FEG-SEM) and Raman spectroscopy were the characterization techniques used. The TDVC coating showed up as an excellent diffusional barrier and, also, a compressive stress reliever. The thicker high quality and adherent HFCVD diamond film grown over the TDVC had a thickness of 3 μm. A TDVC cross-section of 35 μm thickness could effectively mitigate diamond residual thermal compressive stress state after cooling.