"Lots of vapor" isn't really compatible with a Kayfun, unfortunately. I'd probably use 28ga, as I've heard Kayfuns like to be run around 1.2-1.5ohm coils (which is consistent with my experience running a Kayfun Lite+ around 12 watts). At that resistance with a mech, you're certainly not going to be winning any cloud competitions. But again, that's not really what Kayfuns are designed for.
Some people report being able to handle 20+ watts with a Kayfun, which would be around 0.8 ohms. That's probably as low as I'd go, and only if you really didn't like the higher resistances.