Well, I just got out of Vault 3 and Bryce Anders started to talk to me. I saw that you have a speak challenge option there and it seems like you can do this countless times. Which means you can earn infinite xp by just clicking. I won the Speak Challenge Achievement on Steam and advanced to the next level, but it's boring so I went to kill some Deathclaws.

In case this is a bug, I don't want my achievement to be removed

True..

f you independently go to Vault 3 before you even visit a camp McCarran and meet Bryce lying in the vault server room during your conversation you may get infinite experience by asking him "What an NCR ranger is doing here" and then choosing "[SPEECH 25] I'm independent contractor with NCR" you get +25 EXP every time.