At that distance you wouldn't be vaporized, but burned. What you see on the steps is not vapor deposits, but rather they are shadows.

The immense heat and light from the detonation burned/discolored the stones, but not in the shadow of the person sitting on the steps. Hence why you can see these 'permanent shadows' in various places in the city. Some caused by humans, but most are just shadows of structures. For example bridge railings: https://www.atomicarchive.com/media/photographs/hiroshima/im...