The V-Ray lights do not appear in the render: a bug or a matter of lighting setup?
When it comes to lighting, V-Ray’s main objective is to produce physically accurate light - the light’s behavior is developed to resemble that of the light in the real world. With this in mind, a whole range of render settings, as well as the scene setup will affect how the lighting behaves in each individual case. This is important, as the 3D environments we work with (the viewports) do not always translate the information in an intuitive way.
This can sometimes lead to confusing results when it comes to light sources. A commonly perceived issue, upon which even some more experienced users may stumble - the light sources seemingly not casting any light, is in fact a matter of lighting setup.
Why are the lights not visible and how to troubleshoot this behavior?
For the purpose of this explanation, let us imagine the following real-life scenario: A bright day at noon where the light emitted from a flashlight seem invisible when pointed at a surface illuminated by the sun. The much stronger light from the sun is completely "overpowering" the weaker flashlight.
For future reference, please see the following Exposure Values chart.
A similar setup to that imaginary scenario can be achieved in V-Ray as demonstrated in the screenshot below, where the default ~14 Exposure value is used for the scene:
It is evident, that the lights projected onto the cube are either completely invisible or barely distinguishable. To test if there is an actual issue, or if the V-Ray Lights in the scene are simply being "overpowered" by the environment, the Sun's intensity will be set to '0' from the V-Ray Asset Editor > Lights tab.
This is the result:
The lights are now completely visible. They are indeed too dim to show up in a brighter environment.
In such scenarios, the approach is to either adapt the V-Ray Lights to the environment conditions by increasing their intensity or to modify the environment to such one, where more accent is placed on the artificial lights (e.g. late afternoon/evening lighting setup).