The Cherenkov effect usually occurs in a certain medium. In this medium, such as water, the speed of particles exceeds the speed of light in the medium, which will produce observable Cherenkov radiation. If the dielectric constant in the vacuum may change for some reason, such as the increase in the dielectric constant due to vacuum polarization, it may cause the actual speed of photons in the vacuum to be lower than the theoretical speed of light. In this case, even in a vacuum, the Cherenkov scattering effect may occur. In this paper, the data of supernova 1987A is used to estimate the light speed reduction caused by vacuum polarization, and then the energy of the particles needed to produce the Cherenkov effect in vacuum is given. The estimated results are compared with the excess positron data observed by Alpha Magnetic Spectrometer (AMS), and it is found that the two are basically the same. Therefore, the excess positron phenomenon observed by AMS2 can also be explained by vacuum Cherenkov radiation.