18-month fixed term contract – Vacation Questions

Hello, 

I have two questions regarding an 18-month fixed term contract in Ontario. 

1. If an employee is on an 18-month contract, and we pay out 4% of vacation pay, are they owed any additional vacation/time off? 
2. If an employee is on an 18-month contract, should we specify in the offer letter how much unpaid time off they can take? 

I would be grateful for any guidance you can help provide.