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I currently have the 640i convertible on stock suspension, which I imagine to be softer than your M Sport, but I know what you mean about the rear end tramping when cornering on uneven surfaces. I only notice it very occasionally when I'm cornering on bend with an adverse camber and at quite slow speed, but it can be somewhat unnerving.
I bought my car used (1 year old with just 1200 miles) and the deal was too good to refuse (45% off list). Had I bought new I would definitely have opted for the AD+VDC - I had it on my previous car, an F10 535d M Sport and I found it amazingly effective.
As to your vibration through the head restraints - if it's happening with the car stationary, it can only be engine related and it most definitely should not be noticeable. The engine in my 535d is basically the same unit as fitted to your 640d and it should be very smooth at all RPM settings. I also test drove an F13 640d a while back and I found the engine and ride to be remarkably smooth.
If your vibration is happening when the car's moving and you find that the frequency alters with road speed, it will be transmission or wheel related and, as it's coming through the seat, that would most likely point to transmission or rear wheel balance/bearing.
Hope that helps - good luck at the dealer.
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