The G12K100 alone, at least to me, seemed to sound a little bit (really only a little) sterile and were lacking some mids for smooth leads. The Vintage 30 adds those mids without having those annoying high mid ice picky thing going on. Both speakers combined just had that very powerful, balanced and natural sound with nice highs, tight bottom end and just the right amount of mids. The V30 cab in comparison had way to much mids for my taste and was lacking the nice highs the G12K100 adds. Especially with complex chords the V30 seemed a little muffled and you couldn't really tell every note apart. I'm very pleased with the combination.
All based on playing the cab with a VH4. The V30 might work better with the Herbert because the VH4 has more mids.
G12K100s come in 8 ohm only. V30s come in both 16 and 8 ohm versions. Hopefully your V30s are 8 ohms, otherwise I would absolutely not mix different ohm speakers in the same cab.
It'll result in a strange ohm measurement and the 8 ohm speakers will get 2/3 of the power resulting in an unbalanced responce.