I took the "Damn Vault Dweller" quest in Junktown (not the Crazed Vault Dweller from Telltale).
I looked all over between Shady Sands and Reno and could not find the place, even though the log said it was marked on my map.
I went back to the quest giver ("Fairweather", in the soutwestern part of town) and could not retake the quest.
I found the marker- it was way over on the right side of the World Map, 5 squares SSE of Vault 15.
So not a bug, just misleading text.