Last chance department:
Have you tried any 510/eGo or 510/510 adapter on it? Most times adapters have better cut threads that can still get a grab on partially damaged or stripped threads when devices cannot. Not ideal, and they would have to be really careful not to drop it and kill any remaining threading off in the process. If that works a beauty ring - aka a tank support ring - would help to minimize further thread damage by supporting connected devices.
Can you post either a photo of the top cap on this one or a link to the APV on-line? Maybe we can figure out if there is any practical way to get the top cap off. There were/are a lot of knockoffs that use "Lava" in the name so we need to be sure what model we are talking about.
Creative problem solving: If the guts can be removed without damaging the electronics you have the makings for a box mod. Anyone feeling crafty?