Refractive Index of a Vacuum is 1, Refraction Index is represented by symbol n which is expressed relative to the speed of light in a vacuum. The index of refraction value of a material is a number that indicates the number of times slower that a light wave would be in that material than it is in a vacuum.
Also nothing can travel faster than the speed of lightin a vacuum and lightin a vacuumalways travels at the same speed.