I visit the dunes at least 5 times per year. NEVER have I had a problem getting a dune ready voucher. I have not always been able to get the 9-11 voucher but I am always in by 11:30.
They hand out 250 dune ready vouchers every 2 hours. That's a vehicle into the dunes every 30 seconds. Never is the line backed up that bad other than holiday weekends.