all you need is to be able to pass a 25 speech challange when you start the quest to go into the vault that has the venus fly traps and the green spore things. Ok after talking with Dr Hilern and walking out the assistant will stop you and start talking about finding keely. . Go thru dialog till you get one that starts. :Can you tell me about keely ? she talks. Then pick : Exactly how old is she ? she talks. Then pick :What do you think about Dr hildern ? she talks.
then there will be a (25 speech ) challange . pass it and just rince and repeat for a 25 Exp that can be done over and over . Note it can only be done before you leave . You cant do it when you get back .

Easier way. Vault 11. 500 xp. can be made at lvl 1-4. I made it there starting from level one and goodsprings. once I had vault 11 on my map, I headed to Novac to retrieve Boone. Then to Primm, to pick up Ed-E. I was level 2 when I entered the vault and was half-way through level 3 when I entered the final room. Only other thing you need is at least one stealth boy.

yep vault 11 you can go from level 1 to 30 in seconds i use it too test out character builds if i dont have much time

cool thanks for the info guys

Even better way is to do "That lucky old sun" ( I think that's what it's called) and side with the followers of the apoc. Overload the power for the helios solar panels and talk to the followers guy about it. Tell him you overloaded the power and he gives you 300xp, 3 stimpaks and 2 doctors bags. You can repeat this as many times as you want, and even exit the conversation, level up a bit and talk to him again and still be able to do it. Forgive any misspellings or slight misinformation. It's 3:50 am and I really should be asleep right now >.<