California Labor Code §227.3:
Unless otherwise provided by a collective-bargaining agreement, whenever a contract of employment or employer policy provides for paid vacations, and an employee is terminated without having taken off his vested vacation time, all vested vacation shall be paid to him as wages at his final rate in accordance with such contract of employment or employer policy respecting eligibility or time served; provided, however, that an employment contract or employer policy shall not provide for forfeiture of vested vacation time upon termination. The Labor Commissioner or a designated representative, in the resolution of any dispute with regard to vested vacation time, shall apply the principles of equity and fairness.
Per the California Department of Industrial Relations:
Under California law, unless otherwise stipulated by a collective bargaining agreement, whenever the employment relationship ends, for any reason whatsoever, and the employee has not used all of his or her earned and accrued vacation, the employer must pay the employee at his or her final rate of pay for all of his or her earned and accrued and unused vacation days. Labor Code Section 227.3. Because paid vacation benefits are considered wages, such pay must be included in the employee’s final paycheck.
The California Supreme Court has made clear that
[t]he right to a paid vacation, when offered in an employer’s policy or contract of employment, constitutes deferred wages for services rendered. Case law from this state and others, as well as principles of equity and justice, compel the conclusion that a proportionate right to a paid vacation “vests” as the labor is rendered. Once vested, the right is protected from forfeiture by section 227.3.
On termination of employment, therefore, the statute requires that an employee be paid in wages for a pro rata share of his vacation pay.
Suastez v. Plastic Dress Up (1982) 31 C3d 774 (footnote omitted).
California Labor Code §202(a):
If an employee not having a written contract for a definite period quits his or her employment, his or her wages shall become due and payable not later than 72 hours thereafter, unless the employee has given 72 hours previous notice of his or her intention to quit, in which case the employee is entitled to his or her wages at the time of quitting. Notwithstanding any other law, an employee who quits without providing a 72-hour notice shall be entitled to receive payment by mail if he or she so requests and designates a mailing address. The date of the mailing shall constitute the date of payment for purposes of the requirement to provide payment within 72 hours of the notice of quitting.
California Labor Code §203(a):
If an employer willfully fails to pay, without abatement or reduction, in accordance with Sections 201, 201.3, 201.5, 201.6, 201.9, 202, and 205.5, any wages of an employee who is discharged or who quits, the wages of the employee shall continue as a penalty from the due date thereof at the same rate until paid or until an action therefor is commenced; but the wages shall not continue for more than 30 days. An employee who secretes or absents themself to avoid payment to the employee, or who refuses to receive the payment when fully tendered to them, including any penalty then accrued under this section, is not entitled to any benefit under this section for the time during which the employee so avoids payment.