High pressure torsion (HPT) is the way to form nanostructured materials with high strength properties. HPT hardening mechanisms in BCC metals, such as niobium and vanadium, are not fully understood. Here, we have demonstrated that a recently proposed piecewise model can extract from the hardness-strain experimental data the different structural states in BCC metals. The piecewise model, with overlapped stages, is based on an assumption that two separate deformation mechanisms which are responsible for the type of structure have overlapped strain region. To confirm/disprove extracted boundary values, we performed a transmission electron microscopic study of the HPT niobium and vanadium. We found that the piecewise model provides accurate extraction of the boundaries in niobium and vanadium. We proposed an explanation for the observed difference in the structure formation/transformation in niobium and vanadium upon HPT.