The energy of vibrating electrons is re-emitted as light. Hence, glass is said to be transparent to the frequencies of visible light. The frequency of the re-emitted light that is passed from molecule to molecule is identical to the frequency of the light that produced the vibration in the first place. The main difference is a slight time delay between absorption and re-emission. It is this time delay that results in a lower average speed of light through a transparent material.
Light travels at different average speeds through different materials.