Be prepared with good reasons on why you want more time off, and it's best to be honest about it. Rehash your past performance if you're trying to strengthen your case that you are a valuable employee to the company. Try to explain that if you were able to take more days off, you would be more productive and it would benefit your mental health and physical well-being. Talk with your employer about why vacations matter and that you’ll be even more productive when you return to work.
It helps to offer to work extra hours leading up to your vacation and after you return from your time off.