I emailed them on Friday, after I purchased my tickets, and asked the same question. Here is their response (I'm going to follow up in a few days if I haven't received the voucher).
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Dear Sara,
We will send you the voucher once we have the confirmation number for the tickets from
Disneyland.
If you have any further questions, please contact us.
Thank you,
Customer Support
Last Minute Travel®
MV7368
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From: My email address
Sent: Friday, April 02, 2010 10:31 PM
To: LastMinuteTravel
Subject: reservation xxxxxxxx, record locator xxxxxxx
Hello,
I purchased Disneyland tickets online from Last Minute Travel earlier today. The confirmation I received said to present the voucher at the gate for admission. However, no voucher was sent to me. When can I expect to receive it? My understanding from others is that this is emailed separated as a .pdf file.
My reservation # is xxxxxxx and record locator #xxxxxxxx.
Thank you,