@mandeep Identical symptoms to my car. I'd only recently purchased mine as an Approved Used Audi and it was rectified by the dealer eventually.
They did not share much detail, but mentioned that the source of the water was in fact from under the wheel arch, and that the problem is rectified by fitting self draining bungs.
You have no hope of removing all of the water with your vac as it sits in all of the cavities under the floor. Over time this will cause a mouldy smell in the car if not rectified.
It is a big job, as all of the water must be removed from the car, and its possible that trim, carpets and any water damaged components will need to be replaced. This is why Audi will not acknowledge it.
Perhaps the community on here, the fact that there is a class action suit and that Audi almost certainly have a TSB about it will persuade them to listen.
Failing that I'd go to a consumer organisation like Which? and see whether they would be prepared to act.