Here's how they work...
You start with X negative pressure (vacuum) in the tank. 'X' is determined by variables in the air system, the original assembly vacuum and user generated vacuum. In the vent system, we have atmospheric (14.7 psi at sea level) or positive pressure. One pressure balanced against the other... both working to keep the juice in the tank, rather than your pocket.
You go to a higher elevation and what happens? Right, atmospheric pressure drops. At 5,000 feet, a bit less than 20%.
Now... how does that effect a Kayfun or any other
pressure differential atomizer? There is less positive pressure pushing against the fluid boundary between pressures... and to compound matters, our vacuum is also decreasing as elevation increases. At our 5,000 feet, a little less than a 20% loss of vacuum. (Funny how some of these numbers seem to correlate... ain't it? ;-) )
At some point, as pressure inside and outside approach a neutral point, our pressure differential isn't sufficient to keep the fluid from flowing (gravity, fluid dynamics, capillary action) into the positive pressure area of the atomizer.
Two things to consider: The wick used, it's density and volume come into play, as the wick acts as a sort of "leaky seal" between the two pressures. Taking draws, with the vent system covered, will serve to increase (supplement) vacuum in the tank.
The folks that build Kayfuns at high elevations have less to work with in both absolute vacuum and pressure, but the balance is the same, so they have no elevation induced issues. All you have to do, as a "flatlander"... as elevation increases, compensate.