I'm in almost the exact same situation. I purchased two 1 year PSN codes from Amazon and tried to redeem them on my Vita. The first one was accepted successfully, but I'm getting an NP-8154-8 error on the second.
I just did it and I'm downloading Battlefield 4 now.
1. Redeem the code via the website(towards top right under Accounts section select Redeem Prepaid Card)
2. Go back into the PSN store on your PS3
3. Go to the content for the code you recently redeemed and click Download.
4. Enjoy your new content!
That's it, quick and easy. Hope it works for you.
are any of u guys getting those errors now as well?? i bought 1 voucher and another was the one which came along with the purchase of ps vita. Both seemed to be having the same error. is it just me or is something wrong with the system back at sony? please do reply....... I'm going nuts