Fallout 3 help

Hi guys and girls. Im trying to do the "Trouble on the homefront achievement",the one where you go back to vault 101. I know Ive to go back to vault 101 and receive a radio message but thus far I havent received one. So,I was wondering,do I have to be a certain distance into the main story or something?

yes you have to have completed the water of life mission (part of the main story). After that just wander back towards the vault and you should pick up the radio message.

However if like me on my first run through you run amok in the vault before you leave and shoot everybody and everything the vault misison will never appear as the survivors are majorally pissed with you

It does stop being available as a side quest late on in the game. There is a glitch however, that if you complete The Pitt DLC it becomes available again. This is how I got to do it.