to be safe you need to not have air (oxygen) in the system. you don't need to pull a vacuum at all to braze, just purge with nitrogen. You don't want to be heating propane gas (or compressor oil) to red heat in a pipe anyways, oxygen free or otherwise.
a 100% propane atmosphere isn't flammable.
Quote:
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Propane Limits of Flammability - The lower and upper limits of flammability are the percentages of propane that must be present in an propane/air mixture. This means that between 2.15 and 9.6% of the total propane/air mixture must be propane in order for it to be combustible. If the mixture is 2% propane and 98% air, there will not be combustion. If the mixture 10% propane and 90% air, combustion will not occur. Any percentage of propane in a propane/air mixture between 2.15% and 9.6% will be sufficient for propane to burn.
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https://www.propane101.com/aboutpropane.htm
also unless you're holding a torch to your entire system at once, you're unlikely to "blow up" the system. brazing a joint is not the same as holding a torch to a receiver/grill tank.