A major disadvantage of the photon map method is boundary bias, where areas around corners and object edges appear darker than they should be. Other methods can also show boundary bias, but can be adapted to decrease the effect greatly. Another disadvantage of the photon map is that it cannot simulate illumination from skylight because the photons need an actual surface to be emitted from. The skylight, at least in V-Ray, is not a surface actually present in the scene.
On the other hand, the photon map is view-independent and can be computed relatively quickly. This makes it ideal for quickly approximating the scene illumination when used together with more accurate methods like Brute Force or Irradiance map.
The above diagram shows the way the Photon Map is generated. In the first pass (blue) rays are traced from each light into the scene, which generates light samples across the entire scene. In the second pass (red) all samples in a specified radius are gathered from the camera in order to create the final GI solution.
For more information about the Photon Map render Settings in V-Ray, please see the Photon Map Settings page within this documentation.