I'm in SWFL - having done this before (Personalized & Specialty Plate) today went right down to my local DMV with my applications already filled out. Had what I wanted written on the plate, and what plate design wanted for each car.
I did not go the pre-sale voucher route - I waited until pre-sales were completed, the number was reached, and the plate was entered into official production (to which i don't know the date of, I just know pre-sale vouchers were over).
I'd give your local DMV office a call, and tell them you have a voucher, can you go in and get the ball rolling - should be good to go!