The schedulers for each area use a mathematical formula to determine how many cast members are allowed to have a specific day off. Generally speaking, this formula takes into account the number of cast that are statused in that area, park hours, park attendance forecast, etc. Full time cast do a vacation bid once a year in seniority order. If they want to add vacation days, or if part time and CP cast members want time off, it's first come, first served.
If you are waitlisted for time off, it means that your area is already at their cap for how many people can be off on those dates. In order to allow you to have the time off, people who already have the time off must cancel that request. (It happens ... plans change, spouses aren't able to get vacation time, school schedules aren't announced at the time of the vacation bid, etc)
Your best bet is to try and find someone to give away or trade the shift to. If the full-timers in your area are getting less than 40 hours a week, one of them may take a shift, if not, check with the part timers, and seasonal folks.