...I have been doing some research on replacement cat's and if you get them from Isuzu they are not cheap ($1,400 per side). My local shop wanted to charge me about $1,400 to cut the old cats out and weld in new (universal) ones, but after looking around a bit it looks like there are a few companies that make the entire left and right cat assemblies for the VX (Benchmark, Bosal, Eastern Catalytic) and the prices aren't bad (
$300-$400 per side).
http://www.autopartswarehouse.com/sk.../BO099014.html That is one example, but I have found them for less money. These are the complete cat assemblies so there is no cutting and welding involved. You just unbolt your old ones (which might require some heat on the nuts and bolts to get them loose) and bolt up the new ones (with new gaskets).
Hope that helps.