Complex sentence: At the same time, a few guys (one of them producer Butch Vig) from the party dumps a vat of wine over the floor, so now the band is showered with both plaster and wine.
Split into simpler sentences: At the same time, a few guys (one of whom is producer Butch Vig) from the party dumps a vat of wine over the floor. So now the band is showered with both plaster and wine.