I have a salaried employee who has requested a vacation day that is 2 months prior to his being eligible for paid vacation time. Can I deduct 8 hours pay from the pay period in which he takes the day?

What is in your company policy? If the employee has no vacation built up, then no pay for the day of missed work. If you advance pay for the missed day, then you can deduct the pay the next period. If you allowed it before and other people were paid while not earning vacation, then a precedent has been set and you need to tread lightly.