I don't believe it would work. VG music will always be there to accompany the actual gameplay, but there shouldn't be any confusion as to where the true focus lies.
Game music can do many things -- it can set the mood, establish atmosphere, funnel motivation, carry a player through the action, or simply give the game a sense of pace; it's there, of course, to be listened to, and so it keeps the game alive. The stronger the musical theme, the better; the more individuality, the more character it can bring into the game, the more effective it becomes.
But it isn't there to distract you; it's there, if anything, to focus you on the task at hand. it can make playing feel powerful and exhilarating, or pensive and melancholic, but it isn't there to take you away from playing. And yet that's exactly what adding vocals will do.
By virtue of their lyrical qualities, these songs need to follow very specific construction patterns, most of which aren't adapted to the "background" listening video games require. And by their very nature, vocal tracks attract your attention; part of your mind will be intent on deciphering the song's lyrics, rather than focusing on the task at hand. Instrumental music, on the other hand, doesn't have that handicap; it will often translate into a purely emotional impact, giving you that "feeling" of gaiety or dread that often distinguishes a good game.
You might also have noticed that vocal airs usually requires a more directed intellectual effort to appreciate than instrumental music. You won't be listening to an opera in the same way that you'll be listening to an orchestral serenade. Similarly, you won't be listening to a good instrumental CV soundtrack in the same way that you'll be listening to, say, Nightwish.