Poof point is auto ignition point, very bad grease fire. Double boiler won't heat above the temperature of boiling water at your altitude, so no good. You don't need a thermometer, it will only get in your way. Just pay attention and think about what you're doing. First thing is put soap, vaseline, and paraffin in a saucepan on direct heat and melt the waxes. Cut the soap into slivers with a pocketknife. The soap will start to foam, hold the heat at that point (turn burner down) and simmer until the mixture is calm. You should see no oil smoke coming off the mix at this time. Then turn up the heat and start stirring non-stop until all the little grainy lumps of soap disappear. It will be smoking heavily at the point the soap melts, so do it outside and don't breath the smoke. Once it all goes clear as you'll see because you're stirring it constantly, pull the pan off the heat, throw in your beeswax (you can wait a couple minutes for it to cool some before doing that) and keep mixing the beeswax around until it all melts. By that point the mix will be pasty. If need be, put back on low heat to finish melting if not all the beeswax block melted in before it cooled too much to stir. Once the beeswax is in it, don't overheat again, just maybe 250F or so.
I gave temps as a reference only. Here's a different way of describing the process: Heat just above boiling point of water to boil water out of soap, this will take probably 6-10 minutes at constant temp just above boiling water. Once that's done you have to get the mix hot enough to melt the soap, do this quickly on high heat stirring non-stop so that you minimize the stuff lost in the smoke that the mix will start to give off. Once that's done, let it cool it back down and it will start to gel immediately. Beeswax scorches above 357F so you can't melt it in with the other waxes up to the soap's melt point, and it's a good idea to let the grease cool off a little before adding the beeswax. There are other ways to do it if you have two outside burners and some snow, but the way I described will serve you fine.