Is your VT a V8 too? If so it should just fit straight up. My twin exhaust was made for a VY but I ended up getting it as the guy didn't come in to get it. Bolted up nicely on mine.
If your VT is a V6 then it is a mongrel of a job. I put one on a VS IRS V6 and had to modify the sh!t out of it. Although the rear end would be the same on yours it is too much mucking around to cut up the front and make a y piece. Also it made the car really quiet and I thought lost a bit of power too. Same engines so it would have the same effect.
If its a v6 you will have to change the cats, the v6 only have one cat and the vy have 2, just unbolt it from the manifolds and put twin cat set up on it