Unless customer service is of any help, if you got the email, you have to get it via PS3 sign in. Sony is really backwards with this message sync thing as I saw my voucher on a PS3 under my account since the 20th but it never came to my Vita.
I just got in contact with PSN Support and the representative said that they've been having this problem. His only advice was to wait a day or two more, and if I did not receive the message by then, to call back to see what they could work out. I'm not going to go up in arms about a voucher Sony is supposedly gifting me but I would say it reflects poorly on them to send out emails notifying customers of it while not having it available. Bummer too, because I just told my nephew he could rent any movie he wanted to today.