What is the wheel finish? Polished? Polished with clearcoat? Hyper-gloss paint?

Assuming they are polished with clear, I would not sand them down... it will probably not give you the brushed/silver look you are looking for. Also vinyl I'm guesing won't last long with the constant rotational forces/heat/wheel cleaner. Best solution is to have the centers painted - I'm sure any bodyshop could do it for you, or you could take it to a wheel specialty shop. I'd guess around $400 assuming the wheels need to straightening and they don't have to unmount the tires. Powercoating may be your best option but I will be slightly more expensive.
-Cyrus

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2002 BMW X5 4.4i