Avoid Two-Tier Benefit Policy

Q. We offer all employees two weeks' paid vacation a year. If an employee chooses a 100-percent, full-commission pay structure, how should we set her pay for vacation? The employee wants to take her annual pay divided by 52 weeks, but we feel that's unfair to the employees who are on salary plus commission, because their vacation pay is based on their base salary divided by 52 weeks. Is there a correct and legal way to figure this? —R.D., Florida