Ibrahimovic, a former Juve player who has never been forgiven by the fans for leaving the club so soon after their relegation to Serie B, then gave Milan hope scoring his second 30 seconds after the start of the second half.
However, Vastic's last-gasp spot-kick ensures both these two sides, as well as Germany, still have a hope of joining Croatia in qualifying from Group B.
Going into the Cardiff Arms Park game, the Blues top Group B by four points from Bath, with Leicester also retaining a slim hope of qualification if they beat Sale and the Blues lose.
On Tuesday the Plan B drill passed the halfway point to the miners, spurring a celebration in Camp Hope, the makeshift tent village where many of the miners' family members anxiously wait.