I'll take a quick shot at this one. I live at 8000ft. At altitude,
heat is still heat. If you're boiling water, vapor pressure is a
different matter; water boils at a lower pressure because of the
lower atmospheric pressure, increasing cooking times. But the melting
point of ice is the same. So, solder will melt at the same
temperature at 9300ft. as at sea level. The surface tension will be
slightly less, meaning, if anything, the solder will flow better at
altitude, which is why vacuum brazes flow so nicely.
I'm not familiar with your torches or fuel gas. I use acetylene/air,
as well as oxy/acetylene for my work. I can imagine that you might
experience lower flame temperatures because of reduced oxygen in the
thin air, but, I'm guessing here, that you will still be able to
execute your soldering without noticeable difference.
Your vacuum debubbler should work just fine, as it won't have to
work as hard.