I heard a full gas tank is safer than a half full gas tank. The liquid is not flammable but the vapor are.
It seems in it`s liquid form gasoline has such a thick oil base that a simple spark cannot ignite it, it will go out before it can do anything, but when it is spread out like you said in a vapor, watch out. I remember when I was a kid filling a dixie cup with gasoline from our lawnmower and pouring it on a tree stump then throwing a lit match on it that thing practically exploded before the match ever hit that stump, it was like an atomic explosion, mushroom cloud and everything "FROOOOOM!" I think me and my brother lost our eyebrows and the hair in front of our heads lol!. Matter of fact, I know we did.