To obtain V2O5 from NH4VO3, it is sufficient to calcine ammonium vanadate in air at 500-550°C. Pure oxygen and increased pressure are needed only to obtain V2O5 from metallic vanadium (vanadium powder).
Dear Arina Levina vanadium pentoxide is an industrial chemical which is easily obtained from commercial sources.
The following article entitled "Chemical Synthesis of Vanadium Oxide (V2O5) Nanoparticles Prepared by Sodium Metavanadate" is available as full text on RG:
Article Chemical Synthesis of Vanadium Oxide (V2O5) Nanoparticles Pr...
New articles about vanadium pentoxide mostly deal with nanoparticles. In my opinion making bulk vanadium pentoxide is a waste of time as it is cheap and easily available.