No.
In 2014, a California appeals court confirmed that an employer can deduct from an employee's vacation or leave time for partial-day absences in any time increment specified by the employer’s policy (Rhea v. General Atomics, 174 Cal.Rptr.3d 862 (2014)).
Under the salary basis test for exempt employees, employers are obligated to ensure the employee receives no less than the proportionate amount of their salary for a partial day’s absence due to personal reasons, even if a portion of such pay comes from the employee’s vacation bank.
You are never permitted to deduct from an exempt employee's salary for a partial day of absence. Their full day’s salary would be due if the employee’s vacation bank was insufficient to cover the missed daily time.
Read more about Vacation in the HR Library.