If it was only N, I would suggest you to go for heat treatment. But according to you if it is some ion, I would rather suggest you to search some solution which can dissolve those unwanted ions without affecting the desirable solid product. As it is not certain that whether those unwanted ions can diffuse in the solid or just evaporate out during heat treatment.

but you can try heat treatment too if you are sure about the stability of solid compound and well aware of the diffusion kinetics and evaporation behaviour of the unwanted ions.

I agree with the above comment - it all depends on why you want to remove the nitrogen. If you are looking to remove the ammonia, then heat treating should do this, leaving you, in theory, with vanadium oxide, if you drive the reaction to completion. Careful control of temperature and heating atmosphere should achieve this I think. Look in the literature for the decomposition temperature of the compound, if you can't find it then a TGA analysis of your material should tell you what you need to know. Alternatively, if your requirement is other than removing the ammonia, please provide more information so the community can be of more assistance.

Heat trearment IN VACUUM may give better results for this purpose, but making prediction without experimental attempts is quite difficult...

Thank you very much for your advice. I really want to remove ammonium from the heat treatment, but I am considering using a nitrogen atmosphere or air.

I also tried to do TGA test.