Thanks for noticing that Vox is available from the Humble Store at the moment. This is a very new development and I only got accepted and organised with this a few days ago.
I am new to their systems and currently don't know how, or if, it is possible to link Desura, Humble and IGS purchases together and share keys.
I will inquire about this and look into it and try my best to offer you this functionality if I am available.
I know that currently there is no way for me to add Humble keys to the IGS website for me and I dont know if this is something that is planned to be added in the future, maybe mgnade can let me know.
I know Humble Store can include Desura, Stem or even GOG keys into Humble Store purchase...
I know also that its possible to issue gift keys fro game you sell in Humble Store (Paranautical devs made that, for example) but never hears of ny developer selling keys for Humble store and i'm not sure its possible...