Steam occupies much more space than a liquid. A vacuum is created because there is space with nothing in it (well not nothing but close to nothing). Empty space means a vacuum.

Input of energy into water creates steam as the molecules of water absorb the energy,(excite) and force (expel) against each other, thereby taking up more space. When the steam condenses, (energy removed, cooled or escapes), the force between the molecules reduces and less space is occupied. When the steam is contained in a sealed defined space, volume , or container nothing can enter that space including outside air at normal pressure; therefore the container will measure less pressure compared to normal atmosphere, (defined as vacuum).