Another possible explanation for heartburn and acid reflux in vapers I have heard before is due from propylene glycol, i.e. PG. From what I have heard, propylene glycol may be related to or even contain glucose. I have no facts to base this on as it is just something I remember reading before, but some people who were glucose intolerant have claimed that after eliminating PG-based
ejuice and switching to 100% VG-based, their acid reflux instantly went away. Again, I have no facts to back this up, but if you have some 100% VG
ejuice lying around, it won't hurt to give it a shot and see for yourself.
Good luck to you!