Can an employer in Missouri force their employees to take a vacation day if the office is closed because of weather? My employer had to close their office after a snow storm because the office lost power. They are requiring the days missed during the power outage be taken as vacation days.

Yes, that is perfectly legal, as there is no law that prohibits it. If you are an exempt employee, however, you still must be paid for that day if you were out of vacation to use.