What is Vintage Voicing?
Traditionally, guitars equipped with single coil pickups were equipped with 250K pots. A 250K pot will load a pickup so that the sound is softer on the higher frequencies, some might say sweeter. Because a single coil has lots of high end, this is a good thing.
Humbucking pickups have considerably less high frequency response because of several differences in their construction. A 500K pot is usually used so the maximum amount of high frequency is allowed to pass.
The problem arises when you combine the two styles of pickup on one guitar. Either the humbucking sounds are great and the single coils sounds are too bright, or the opposite happens. Vintage voicing is a passive circuit that is switched on and off automatically so that the single coil pickups think they are seeing a 250K pot while the humbucking is seeing a 500K pot. This way no compromise is made on either sound.