I received a 1099-K for the gross payment of "real estate" rentals of vacation club reservations. How do I enter my expenses (annual maintenance fees) for those rentals?

It depends. There is a minimal rental use rule explained below. If that doesn't apply in your situation then it will be income.

Minimal Rental Use

There's a special rule if you use a dwelling unit as a residence and rent it for fewer than 15 days. In this case, don't report any of the rental income and don't deduct any expenses as rental expenses.

If you do not meet the minimal rental use rule, then you must report the income on Schedule E because it is considered real estate rental (assumes this is a timeshare unit).  There is nothing to depreciate because you do not own the property.  Other expenses such as your annual maintenance fees (rental portion only if there was any personal use) can be used as an expense.