Is it possible for 1 litre of vodka 40% alcohol volume to be lethal for an average size man?

Yes: If that person were alcohol naive, or if it were consumed very rapidly, or if there were other sedating cns-acting drugs on board, yes. Every fall during the unfortunate 'hazing' season at fraternties/sororoties many young people die of significantly less alcohol that that, because it is rapidly "chugged.'" most people fall asleep or throw up before alcohol will reach that toxic a level.