So i can only use 245/30R20 to not get defected? Also are 20's are legal on 2004 VY S Series 2?
Yes and Yes.
If you look on the inside of your drivers door there will be the tyre placard. On the placard it will have a list of wheel/tyre sizes and load rating. Any tyre you use must match or exceed the lowest rated tyre on the tyre placard. I think your car will have 235/40R18 listed with a load rating of 91 as the lowest one, but go have a look anyway.
Nankang 245/30R20 for instance have a load rating of 95, thus being legal. There are some other (more expensive) brands that also make tyres in that size and have a suitable load rating.
There is a lot of misinformation about what size wheels are legal and what not. There used to be a 2" rule where you could only fit wheels that were 2" larger than the biggest factory wheels, so if say 17" was the largest factory wheels for a car, 20" would be illegal, but that law is now defunct. Even if it was still a valid law you would be fine because VYs came with 18s.
The new laws say that the overall rolling diameter of the wheel/tyre must not be more than 15mm greater then the largest wheel/tyre listed on the tyre placard - therefore if you put big wheels on you must use low profile tyres, which makes sense. The standard 235/45R17 on a VY S-pack is 321.65mm, and a 245/30R20 is 327.5mm, so you are ok there
There are also laws regarding wheel offset/track, but:
a) i don't think anyone even bothers to enforce them
b) if you had it wheel with enough offset to violate the laws, it probably wouldn't fit on the car with 245 wide tyres