wiseco has stopped making the forged big block kit piston. Vito's forged big block pistons were made by wiseco and they are no longer manufacturing them which is why they're out of stock.
The cast piston is an OK choice. Forged is a better piston hands down but if you can't get forged, a cast piston is better than no piston at all!
There is another choice but it's more expensive and not properly shaped for the ports like the vito's is. The Wossner big bore piston will work in the vito's cylinder but it only has one intake window located in the back instead of two. There are several members on here running the wossner in a vito's kit with no problems at all and the wossner piston is forged!
It's up to you whether you want the cast piston with the dual intake windows or the forged piston with the single.