I don't have anything helpful for you - but I sympathize. For some reason, though - or, probably, for no good reason whatsoever - that stuff bugs me more in pop songs - I was listening to Donovan do Catch the Wind the other day; I'd forgotten about the "la, la, la" bridge; my thought was - why didn't he make up some words there, did he just run out of steam? But I'm sure that's someone else's favourite part, so, what can ya do? What I do in pop songs, usually, is to play the vocables part on harmonica, but in the older folk songs, they almost seem an essential part of the lyric. Good luck ... !