Yes, your employer can decide when you can take vacation.
No. Your employer must pay you all your vacation pay before you begin your vacation.
If you take your vacation at two (2) different times, your employer only has to pay you for the time you are taking.
For example, you have three (3) weeks paid vacation. You decide to take two (2) weeks in the summer, and one (1) week at Christmas. Your employer must pay you the two (2) weeks in the summer, and the one (1) week at Christmas.