I cannot answer why VS did this, but I would like to comment that had I been on that plane, I would have been slightly envious but not begrudged her the treatment. Now, I’m no fan of madonna at all, but love her or hate her she is very successful. She probably travels very frequently and most likely every time with a small entourage so is a valuable customer.
Why the “special” treatment. First, I think no airline would put a celebrity of her standing (nor many minor ones for that) in a bus with all us lesser mortals for the simple reason it could endanger her and other passengers should fans try and jostle forward to get a closer look, photo or autograph.
Secondly, her “people” had probably arranged alternative transport so she did not need to use the VS provided bus to LHR (which is most likely what you were all waiting for and probably the terminal was full as well). Third, she has a well known face, and would not be a possible illegal immigrant to the UK, so VS would have vouched for her (if that was even necessary) which allowed her to go straight through and out.
Finally, it is possible to get the same sort of VIP service from many airlines. You just have to be a premium passenger and pay for it. Some advertise it openly, such a TAP, others do not and it has to be requested. It’s quite pricey, especially if you’re with a family, but I find it worth every cent in some out of the way places where you are whisked from the plane, through formalities to land-side and into a car or taxi!
If you are HON status with Miles and More you will always be met at the plane on disembarking and assisted through, so if you don’t want to pay for the service notch up the miles (600,000 in two years)!!!