When you exit the vault go up to Megaton and go in. Lucas Simms will talk to you, be nice, he'll say he like you then defuse the bomb.

Got o your house and get the junk in your house. Go to Lucas Simms house at the top of the hill.
Steal his rifles and nucacolas. Now exit Megaton and go straight, put all the crap you scounged into the vendor and save your game.

All the junk that you put in there should still be in there. Go back to Megaton, steal crap from anything (just don't get caught) and sell your stuff to the caravan outside.

Keep the stuff in the vendor, now goto the brass lantern and steal the money in the floor safe, and now you have about 500 caps.

Keep the stuff in the vendor and whenever low on caps, go to your vendor (hint: keep valuable stuff that you can't use so you can get it and sell for caps, when needed).

It works, i do it to hold Junk.