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I have a vacuum leak somewhere along my air intake. I researched it and the best way to find it seems to be the propane test where you attach a couple of feet of hose to the nozzle of a propane torch and move it around to see when it starts getting sucked as the engine changes the sound.
My question is, is this a fire hazard? I was told that doing a similar test but with brake fluid or similar could cause a fire.
Anytime you use a combustible fuel in a manner not prescribed you are creating a fire hazard. The thing about using the propane tank (we're talking about a small propane torch, not hooking a hose to your grill's tank), you are using just enough which would change the pitch of the engine while it's running. Used responsibly in this manner, it can be done safely. The minimal amount of propane used will disperse in the air rapidly and will not be enough in concentration to support combustion (too much air to start a fire).
Something else you could try if you don't like the idea of using propane is to use a spray bottle full of water. Spray the water into the area where you are checking for leaks and you'll also get a change in engine tone if it finds its way into your engine. The reason this works as well is, if you have a gasket leak, the water will tend to plug the leak for a bit, which reduces the amount of excess air, which changes the tone. Secondarily, water is hard to combust, so when it runs through the combustion chamber while running, it causes the change in tone, too.
I wouldn't use brake fluid to do it (causes other issues), but brake or carb cleaner can work just as well or better than propane. Not as explosive, but will act pretty much like propane does when it enters the engine.
