Did you use oil designed for a wet clutch? If not it will make your clutch slip. I like ATF, but other oils will work if it is designed for a wet clutch.
Does the vito's crank have hot rod connecting rods? I don't think they do. Hot rods are the best connecting rods you can get. I would rather have hot rods. IMO
I don't like spacers, if you add spacers to the bottom of the cylinders it will throw off all your port timings and your motor will run like ass. Just get the 795 pistons. IMO
Get rid of the spacer plate, they throw off port timings. There is NO WAY you need that big of jets unless you are running methanol. You need to figure out what is wrong with your set up.