That's weird.
Two different private keys can generate the same public address, but first time I'm seeing a private key generating two different public addresses.
No. It's impossible to have a single address being derived from two private keys. It is however, possible to have 2 keys derived from the same private key.
How did you generate the vanity address? If you generated it using an online website without using split-key generation,
DO NOT use it or send anything to it.Can you go to bitaddress.org and go to Wallet Details. Key in your private key and see which private key corresponds to your address. Its best to do this offline.