Help with Case When statement

We need to derive a Vacation Tier based on either the Assignment, Job, or Years of Service for an employee. It needs to first look at the Vacation Tier field on the Assignment and if the value is not NULL, then the report should return whatever value is there. If it is NULL then it should look at the Eligible for Self Managed PTO field on the Job and if the value is Yes, then the Vacation Tier Derived column should return Tier3. If that value is No or NULL, then the logic should look at their Years of Service