So far I've figured out that you have to ask the Junktown trapper about "something special", and not the pre-war facility. This sends you on a chase to hunt down another trapper, first at the Adytum salvagers, and from there to Necropolis. At Necropolis I got the name "Wally" from a ghoul in the Watershed--then another ghoul at the Motel tells me to look back at the Watershed for Wally. Once you talk to the ghouls they won't give you a dialog window again, so the quest could be broken at this point.