When you say "blast it out the vapor port" do you mean transfer butane to another tank and use an air compressor or something to blast it out? Or will this work with butane still in the tank?
A short blast to atmosphere.
Butane can hold up to about 32ml/L water (temperature?), but it doesn't like to, so the water falls out and accumulates in the bottom of the tank just below the dip tube, eventually venting rusty water into your extraction if you let it get deep enough to reach the dip tube.
If you invert the tank and let the water settle out and pool at the bottom, when you oven the vapor valve, it will drain from the bottom and vent the water first, before blasting liquid butane.