Mandatory Overtime

In a manufacturing setting, we pay time + 1/2 after 40 hours of work.  Current work day equates to 9 hours, Monday to Friday.  For those on the day shift, a 3 hour Saturday shift has been added to the schedule for some Saturdays.
For employees who have pre-approved vacation, falling within the Monday – Friday range and ending on Friday:

  1. what is the requirement for the employee to work on the Saturday 3 hour shift? 
  2. if the employee does not report to work for the Saturday shift, can the employer take those 3 hours from their vacation or personal time?

We want to promote attendance on the 3 hour Saturday shift, but at the same time do not want to be unfair to people to are away on vacation (which they would have booked prior to knowing that there would be a Saturday 3 hour shift).