Ok the first thing you need to do is find someone to machine your VJ11 turbine housing to fit the VF12 turbine.
For me that was easy as I am one of the design draftees at a mining equipment manufacturer.
So I went to the most senior machinist who is a decent guy with the VF12 turbine housing and a spare VJ11 housing to see what he could do.
There are two areas of concern, obviously the bore of the turbine housing needs to be increased which you could even do with just a hone.
But there is another area to modify as shown here. Sorry about the orientation, for some reason I couldn't rotate the pics with IMAGESHACK.
The bore for the VJ11 turbine sits about 16mm form the back face. This is to clear the blade tips.
Whereas the VF12 turbine bore sits at about 21mm. Larger blade tips on the VF12.
And here is the finished product after a quick wire brushing the morning I picked it up.
See the area where the end of the bore had to be deepened.