If a plane has some windows broken mid-flight (ie open) will the plane suddenly be full of water vapor because of the difference of pressure?
Water vapor is invisible. I think you mean fog - fine water droplets condensed from water vapor.
Pressurized planes fly with an 8000 foot equivalent altitude and humidity in the cabin is low. But, at 35,000 feet (called Flight Level 35) it is likely one would get a brief fog. If loss of pressure is fast, you would only get to watch for a few seconds. At those pressures, oxygen actually rapidly diffuses out of the blood in the lungs and unconsciousness comes much sooner than one would expect. There have been plenty of accidents dating back to the WWII fighters that cruised at 40,000 feet where a pilot thought he could manage briefly without his oxygen. Something like this likely happened to the missing Malaysian aircraft.