My employer is making me take vacation time to go to a medical evaluation with their chosen work comp doctor. That doesn't seem right. Can they do that?

Unfortunately, that is true.  Under Section 287.140.14 RSMo., the employer may require an employee to use accrued vacation or sick time to attend medical appointments, physical rehabilitation or medical evaluations during regular work time--even if they are scheduled by the employer or its workers' compensation carrier.  This does not apply to medical treatment you receive when  you are out of work and receiving temporary total disability benefits.  It only applies to appointments that are scheduled when you are back to work after an injury.