In part it might depend on any language in the terms and conditions of employment dealing with vacations.
For example, if the 15 days was credited or accrued at the start of each vacation year, and there’s nothing in the terms and conditions of employment that indicated these days are conditional on working the whole year, then there might not be a basis for prorating the 15-day entitlement.
However, assuming the employer is entitled to set the vacation pay owing for the current year, as 7.5 days, employers are not allowed to recover this from any other amounts owing to an employee, without the person’s explicit written permission.
If the employee is not willing to co-operate, then small claims court is the employer’s only other recourse.