Pete,
Try adding a numeric attribute DayCount with default value of 0 and the following rule to object VacationRequest:
If BUSINESS_DAY_FORWARD(VacationRequest.BeginningDate, VacationRequest.DayCount) <= VacationRequest.EndDate
Then INCREASE VacationRequest.DayCount BY 1
Tick the While Semantics checkbox on the Advanced tab for the rule. This will force the rule to execute repeatedly until the calculated date moves past the vacation end date. See the While Semantics sections in the User Guide for details on how it works.