I would have to do a more exhaustive search into this but I think the two words are linked together to suggest that the very act of repetitive prayer in itself is vain, especially when it takes on a "performance" value as we often see and hear about.  Jesus did not teach us to pray that way so I have difficulty understanding how/why the practice is so widely embraced... tradition I suppose, not begrudging anyone as far as I'm concerned you can pray any way you want, as long as it is sincere I suspect God's magnificent grace will be entirely forthcoming.