GHS says:
 
Our warmest sounding strings are created by winding 85/15 copper zinc alloy over a hex core wire, to result in an incredibly rich tone. Vintage Bronze are perfect for those overly bright guitars that need to be reigned in, or when you want a deeper tonality to your guitar.
Why Should You Play Vintage Bronze?
You want to bring a richness and tonal depth to your guitars
You're looking for a deeper tonality to match the music you play
You have an overly-bright guitar that needs to be reigned in