I vaporized with an (accidental) double dose last year and killed each and every bee's in a 4 frame cluster in 24 hours. I don't know what the retreatment interval would be safe, but vapor can have acute toxic effects with a relatively mild overdose of weight per hive. Makes me think that rapid vapor retreatment might be very dangerous. Vapor leaves a white to cream vapor deposit throughout the hive. Until the deposit vanishes, I would think that retreatment might overdose the hive.
Randy Oliver's "scientific beekeeping" site indicates that the vapor deposit is just another route into acidifying the body fluids of the bees, dispersing very fine dry crystals that are consumed by the bees.
That said, I am interested in combined treatments, because I have no real brood break. I live in mild winter Central California, and Eucalyptus is now beginnning to bloom. Major flow by January. The oxalic dribble doesn't kill brood under caps, so a retreatment in some form is necessary.
I am thinking that a formic acid treatment using Mite Away strips might be the best over-treatment if I time it for a couple of warm days in a "January thaw" that would allow the Formic acid to outgas. My hives have reduced entrances this time of year, so the recommendation to increase ventilation during formic treatment is problematic.