This is crazy that Bethesda would make a sidequest like this that has you returning to Vault 101 a limited oppurtunity if what I read about it is true. I have the GOTY version and I've already completed the main quest and never got the Vault 101 signal when entering Megaton or hanging around Vault 101. The door is locked and it says the password has been changed. Any other way to activate the radio signal so I can do the quest?

Something's gone wrong, didn't you get the radio signal while circling around Megaton?

Guess you will have to get it on your next playthrough.