This is a highly individual decision which depends greatly on your work arrangements, how much school will affect your work, and how well you get along with your boss. Bear in mind that a boss's job as well as yours is to act in the best interests of the company. It's easier if your going to school has a minimal impact on your job performance. Bear in mind. It's easier if your going to school has a minimal impact on your job performance.
Employers also vary greatly in their flexibility regarding time off.
Be creative in using your vacation time. You might need only half a vacation day to attend an extended clinical or you might skip lunch to leave an hour early for last minute exam preparations. Save some vacation time for school activities that might otherwise conflict with your work schedule. Use your lunch hour to study or even to get in a little lab practice if your job is close enough to school. Students who are already working in the health care field have a distinct advantage.