You can order the DDE with just the voucher. As far as the dates, I called on Friday for my April trip and the CM told me that if I ordered the DDE Friday (3/1) it would expire 3/31/08. However she said the date changes on Tuesday, 3/5 to exprire on April 30, 2008 so I am obviously going to wait until Tuesday to order.
So for a May trip I would call in early April and see when the date changes so you can get the extra month in case you go back next year.
Oh and if you are worried about getting it in the mail in time the CM said I can pick up a temp card at Epcot. They say to allow 6 weeks for processing and delivery but in the past mine has come much quicker and I didn't have to get the temp card.
HTH
TJ