Velocity of Light in Vacuum is defined as the ratio of distanced travelled by light in vacuum to the time taken to do so. For Example: It takes 1/299792458 seconds for light in vacuum to pass 1 meter.
Mathematically,
Velocity of Light in vacuum = distance travelled / time taken = L/T.
Dimensional Formula of Length = M0L1T0
Dimensional Formula of Time = M0L0T1
Putting these values in above equation we get,
Dimensional Formula of Velocity of Light in Vacuum = M0L1T-1
SI unit of Velocity of Light in Vacuum is ms-1