Don't forget that Vasquez was the one that was last and Gorman went back for her, so even if he could drag her and use the flamethrower, he would still have been slow enough for the Aliens to come from the front via a vent in the floor of the shaft, like they did in the movie. So they still would have surrounded and using the flamethower to successfully kill a xeno would mean trying to navigate through burning corpses. I don't know how much difference it would make, I mean they are scared of fire but they were also relentless.