The best practice is to have language in an employee handbook that states either previously granted vacation will only be rescinded upon the mutual consent of the employee and supevisor, and/or that vacation can be rescinded upon the occurence of some type of emergency (e.g. shortage of staff due to the swine flu). Absent this language in the handbook or vacation policy, the unilateral decision to rescind a previously granted vacation leave can possibly open the door to Pandora's box. I'm sure you know that if the process, or guidelines, for rescinding previously approved leave is not clearly expressed in a written policy, you better make sure such decisons are made consistently with past and future practice.
Also, probably just as much a concern as anything else, you have the employee relations problem associated with the cost to the employee becasue the leave is now being denied. You might consider sometype of reimbursement, but again, keep in mind, that if such a policy is not expressed, you are creating a new policy that must be consistently with past and future practices. My only advice without knowing anymore; be careful with these types of decisions.