Well how much are you going to bet the next Gran Turismo is going to be a carbon copy of GT4, with no new tracks, no new cars, and GT4's graphics? And how much are you willing to bet that the new GT is going to be bundled up and marketed as "Vision Gran Turismo?" Vision GT is nothing more but a concept of GT5, starting with more cars on track and more activity in the pits. There's also rumors of car damage according to IGNore (ign.com).
I seriously doubt GT5 will be "GT4 vol. 2." Even when it's about 4 months after GT5's release and some "n00b" revives this thread for no apparent reason (granted it's not locked by a Moderator), I can still say that Vision Gran Turismo is just like the Bentley Speed 8 in GT Mode- Not For Sale. Dig me?