I am a fulltime employee in the state of missouri. I have been required by my employer to document 8 hour days of work regardless if I work more or less than 8 hours a day. I'm a monthly salaried employee and am not paid for more than 40 hours a week but if i work more than 40 cannot document this nor do I get to take the time off. If I work less than 40 hours a week I have to use vacation time regardless. Is this legal?

That depends entirely on whether you are in an exempt or non-exempt position. Saying you are a salaried employee doesn't tell us anything. "Salaried" is merely a pay method - it has no legal standing.

If you are an exempt employee, then you need not ever be paid for any hours you work over 40 nor is comp time required. And yes, your employer may debit your vacation time if you are absent from work.