Just a warning. My friend is a huge Playstation fan. Pretty much exclusivly has all Sony consoles. So tonight I called him up to activate my PSN voucher since my PS3 is busted and my account is on his PS3 so I could find a work around to getting Valkyria Chronicles 2 on my Vita. He didn't even know about the $10 dollar voucher but was happy to do it over the phone. Turns out it is one voucher per console. Now he is SOL and I feel pretty **bleep**. Thanks Sony.
tldr; One voucher per PS3 console. Not per PSN user. PS Vita users are SOL.
You should've had your friend give you details of what he saw on screen in redeeming your code. Otherwise he'd tell you that when entering the store, he saw your voucher code, which he could've told you, and in fact CAN be redeemed on a Vita. I just got mines from my brother's PS3 using my login information.
You should help your friend out now by logging out of your Vita, sign in with his account, and redeem his code for him since he can't use another on the same console.
BTW, why my code was sitting in my inbox since the 20th and never given on my Vita is pretty lame of Sony. Not to mention messages that I deleted off of my Vita is still on my PS3 account.