In California, vacation pay is considered to be another form of wages and as such cannot be taken away from an employee under any "use it or lose it" scenarios.
Employees should make every effort to plan vacation time well in advance to lessen the likelihood that they will not be able to use their time. Make every effort to cover your work if you need to take time off on short notice or as a deadline for using vacation is approaching. If an employer requires you to work during a planned vacation period due to unforeseen work demands, negotiate with your supervisor for a carry-over of your time or some accommodation and ask them to put it in writing.
In many states, employees who terminate employment (or are fired by an employer) are legally entitled to payment for any vacation time which has been accrued prior to any "use it or lose it" cut off dates.