I just had another thought concerning this topic. Its possible that the balance between vocals and instruments shifts depending on genre. I listen primarily to progressive stoner metal, so naturally the music is hugely important in that it has to convey the concept the artist has in mind.
But on concept albums vocals play a huge role in getting the story across. If you listen to the instrumental version of "oblivion" by Mastodon, you'll certainly feel the vibe and be very moved by the music. This gives you an abstract picture of emotions and "colors" if you will. Then you listen to the final production version with vocals and now we have a character that is feeling these emotions and we're seeing the colors form a picture of the situation he's in ("i flew beyond the sun before it was time, burning all the gold that held me inside my shell" and at the end of the chorus "now i'm lost in oblivion")
so before the vocals, we have this feeling of immense isolation and longing. Like being lost in a beautiful landscap. Then here come the vox and we see this boy who flew too close to the sun while astral projecting and it burned out his intestines and the golden umbilical chord that connects him to the material realm. as a result he's floating helplessly into the void.
wow well there's an annoying block of text for ya. hopefully i added something to the discussion with it! XD