D.C. Mun. Regs. tit. 9, r. 9-139
If supplemental wages (such as bonuses, commissions, or overtime pay) are paid at the same time as regular wages, the tax to be withheld shall be determined as if the aggregate of the supplemental and regular wages were a single wage payment for the regular payroll period.
If supplemental wages are paid at a different time, the employer shall determine the tax to be withheld by aggregating the supplemental wage either with the regular wages for the current payroll period or with the regular wages for the last preceding payroll period with the same calendar year.
If an employee receives vacation pay for the time of a vacation absence, the vacation pay shall be subject to withholding as though it were a regular wage payment made for the payroll period or periods which occur during the vacation.
If vacation pay is paid in addition to regular wages to an employee who forgoes his or her vacation, those payments shall be treated as supplemental wages.
D.C. Mun. Regs. tit. 9, r. 9-139