When we burn fuel, water vapour is produced because hydrogen is burnt as well. But since the vapour is in gaseous state already, it shouldn't steal any heat from fuel gases, should it?
Water vapor produced during the burning of hydrogen-containing fuels should not steal any heat from the fuel, unless they condense on solid surfaces. In such cases, it may require a small amount of heat to evaporate them again. However, in my experiences, there is a stage during burning that there is a lot of water vapor on the solid object, and as the object is warmed up, there is no more vapor condensed.