Write a letter using snail mail.
Normally you accept the compensation package, here consisting of the $400. voucher and a rescheduled flight (the next day). If you had wanted something different (such as a prorated refund of the segment) you should have proposed that at the moment of volunteering.
I do know that Delta is one of the airlines that slots volunteers into an intricately prioritized list. But I only wish that they would give me the chance to volunteer for the package announced over the PA before negotiating with later arriving persons for more esoteric items like dollar value vouchers instead of free trips.