Currently the VCOs own 20% of the shares, which is 20% x 1,800M = 360M shares.
Their bonds will be replaced by 600M shares, so they will then own 600 + 360 = 960M shares. The total number of shares increases to 1800 + 600 = 2,400M
So they will then own 960/2,400 = 40% of the total shares i.e. an extra 20%, twice as much as before.
Since the total value attributable to the VCOs is $18,458M, the value due to owning more shares is 50% of this.