Elementary gen chem books will make the careful distinction between the two theories and then proceed to claim that chemists can use both to complement each other to explain the behavior of certain electrons. The classic example is Benzene, held together by a $\sigma$ bond framework with delocalized $\pi$ orbitals. In general, resonance uses MOs and VBT describes the framework to hold the molecule together.
What is the justification for utilizing both theories at once? The construction of the wavefunctions are dramatically different. My only guess is that you need the concept of hybridization from VBT and afterwards, you are constructing molecular orbitals that are superpositions of various hybridized orbitals and other atomic orbitals, is this the right idea?