There is no such thing as a true vacuum. Quantum chromodynamics can show why that's true.
Either way the speed of light is the speed of light in a vacuum. While not in a vacuum, the speed of light is c/n, where n is how light propagates in that medium.
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But if you were in a true vacuum
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Warp Drives are being discussed right now as a plausible way to exceed the speed of light. It works by warping space around an object via a ring of exotic matter, thus creating a perfect vacuum inside the ring. Check out this article for more info on Warp Drives: [url=http://www.zmescience.com/space/warp-drive-plausible-nasa-research-312031/]NASA: ‘warp drive is plausible’ – experiments under way [/url]
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It's more about moving the space around you to "exceed" lightspeed than actually exceeding lightspeed.