Ok I brought this up on Vapor-Info and a guy there told me that this problem has arisen before. It seems that if you hold the unit upside down whilst turned on with the fan set to H, and tap the fan casing a few times, it gets going again. I've tried this, and it has worked so far- and when it hasn't, leaving it for a bit and trying again later seems to do the trick.

I can't think what could be causing this, but it seems a bit shit when I can go to the local cheap shop and buy a pocket fan for £1 which never stops working until the battery runs out, and yet vapir can't get their fan to last more than a month. But at least I'm getting some of my money's worth out of it before it dies altogether.