How is energy dissipated in a travelling em wave. Will there be any dissipation if it were to travel trough vaccum ?
Individual photons will not lose any energy as long as they do not interact with any other particle. If you are referring to the intensity of the EM emission, that depends of the angle incidence from their source.
So basically, if you imagine a laser that could emit just a single photon in the vacuum of space, that photon would maintain its frequency, and therefore its energy, up until the moment it interacts with another particle.
Where $E$ is the energy of the photon, $h$ is Plank's constant, and $f$ (or alternatively $\nu$ is the frequency of the photon.