At what temperature did you sinter them? What is the density relative to the theoretical? If it's 90%+, then you probably can't do much else, since it's already sintered pretty well. If it's 80% or lower, then you can increase your sintering temperature or sintering time.

Try to obtain/procure/prepare ultra fine single phase Vo2 powders. Press them and using conventional solid state sintering prepare ceramics, and measure their density accurately to obtain good repeatability. Approach closely to the thoretically expected density. Look at the fractured microstructure by SEM, and see that there is no porosity, and obtain a good uniform grain size. You may need to optimise your sintering schedule (time and temp ) carefully to obtain the best highest density.

K. Sreenivas

It might help to consider hydrothermal sintering, with vapourized water; combined with added equimolar amounts of isopropyl alcohol and acetone, to avoid net interconversion between V2O5 and V2O3, accordingly: V2O5 + 2(CH3)2CHOH ↔ V2O3 + 2H2O + 2(CH3)2CO. Vanadyl(V) trihydroxy, VO(OH)3, seemingly could occur and play an intermediate role (*), what could, in principle, facilitate either sintering and the intended equimolar conversion of both oxides to VO2.   

(*) You may want to check also my post elsewhere at this forum: https://www.researchgate.net/post/How_to_prepare_a_paste_solution_of_60V2O5_and_40Na2SO4_for_coating_to_do_hot_corrosion_analysis