DOL stands for Department of Labor. They oversee, among other things, any wage and hour issues.
I am unable to locate a statute number describing what must happen to vacation pay at termination, which means that it is probably established by case law. Although my sources generally provide the case name in that instance, this time they did not.
The following is a fine-line distinction, but it is going to be very important to you. I have checked several sources, all of which agree to the following:
1.) If there is an established policy or an agreement between employer and employee saying that vacation time will be paid on termination, then vacation time must be paid at termination;
2.) If there is an established policy to the effect that vacation time will NOT be paid at termination, then vacation time need not be paid at termination. If the policy states that vacation pay will be paid when x happens but not when Y happens, then it must be paid when x happens and need not be paid when y happens.
However, none of my sources are clear as to what happens when company policy is silent on the issue. One source implies that vacation time MUST be paid unless there is a policy saying it won't; another source implies that vacation time NEED NOT be paid unless there is a policy saying it will. As you can see, in the absence of a written or commonly followed policy that can be very important.
My suggestion would be that if you find that there is no written or established policy one way or the other, you contact the state DOL and ask them for clarification. The phone number is 1-405-528-1500 ext. 200.