I've read that people use bio washing liquid in their Vaxs, if you use it how much do you put in? We've just bought a new Vax and while it is good, the patches of carpet I have previously cleaned with bio are still cleaner, so thinking of using it in the Vax. One problem, as it's brand new, and it says on guarantee that using a cleaner other than the recommended on invalidates the guarantee.. so what would you do? Take a risk & try the bio or just buy more cleaner?