He is looking to create brainwallets, not custom vanity addresses. Edward, does it have to be in C++? You could always download a brainwallet script and run it offline. It has the same result as offlinebitcoins.com every time. If your interested, I recommened using
this brainwallet source code, which is a branch of the script offlinebitcoins is using. I haven't checked to see if it will steal your keys, so make sure to only use it offline. I don't recommend using brainwallets though, since they can be easily bruteforced if you pick a fairly easy phrase.