The VX transmission is from GM, so it's nothing special. a back flush is a very good way to replace the fluid. they push new fluid through the trany cooler line, it goes through the entire trany and the old fluid gets pushed out. I've never heard of it being done with the old fluid. I've done a lot of flushes the old way and never seen a filter that was clogged, but if your fluid is almost black, then yes I'd change it. and not taking the pans off means no chance of leaks.
Why?..... because stock sucks.