Vullaby's trivia / origin entries could be updated with an excerpt from the following Ken Sugimori quote from the May 2011 issue of Nintendo Dream Magazine (referring to both Vullaby and Mandibuzz):
“These two were conceived of pretty late in development. When the Englishman came to us with these designs, we already had plenty of birds, but we ended up using them as counterparts for Rufflett and Braviary. The nest around Vullaby’s body come from the bones of other Pokemon, and she grows up inside of them. When Mandibuzz is incubating a Vullaby egg in its nest by sitting on it, it fits together with Vullaby’s bones and looks like it’s sitting on an upside-down skull.”
Source: http://lavacutcontent.com/ken-sugimori-nintendo-dream/