CORRECTION: An adjusted Chart 3 is shown below. You'll see a p2tog symbol at the base of each column of black and purl sts. The pattern changes up the back of the vest, from all cables, to some cables and rib, then to more rib than cables. So, in order for the rib to grow out of the cables evenly, the center 2 sts of the changing cable needs to worked as a p2tog (on the RS, which is a k2tog on the WS). This decreasing will also occur in Chart 4, although the p2tog's will occur on row 1 of chart 4 for sizes S and L, but on row 3 for size M.
Adjusted Chart 3 (This is a jpeg. Click and hold on it to select download to your disk from the menu.)