If I may refer you to Largo's market update teleconference -...

If I may refer you to Largo's market update teleconference - https://seekingalpha.com/article/4264462-largo-resources-ltd-lgorf-ceo-mark-smith-q1-2019-results-earnings-call-transcript. Terry Perles provided a good explanation about the recent dynamics of Vanadium price, including topics such as niobium substitution. $15 is seen as the equilibrium level with balance between supply and demand.

There was also some previous discussion in this forum about this:
https://hotcopper.com.au/threads/terry-perles-describes-vanadium-market-in-largo-conference-call.4766203/