One of the more agonizing things about being a lighter is finishing a shot to render but getting back files that are unusable due to unaesthetic flicker. While the flicker can be fixed by increasing render settings globally, that method comes at the cost of increased render times. Instead, narrow down where the flicker is coming from and solve the issue at the root instead of trying to "throw more money at the screen and hope for the best".
Super important to render things out in passes so that not only will you have control over every single aspect in comp but it breaks the render down into its elements for trouble shooting.
The Pass column is the pass that you want to check if the flicker is coming from. The material column is the source of the flicker. The Flicker column is what needs to be changed to resolve the flicker. If the flicker seems to be coming from the lighting pass, the source is the diffuse of the material, and what you want to do is increase the light rays for the shadow. If the flicker is in the specular pass, increase the subdivisions of the light.
The exception to the above is hair. Hair shows up in the reflection and refraction passes but the VRayHairMtl doesn't have subdiv attributes to make adjustments on. In the case of hair, to resolve flicker, what instead needs to be done is increase the subdivision/fine tune the GI. This is because the flicker shows up in the refraction pass which is a secondary light ray. As there are no refraction subdivision settings in the VRayHairMtl, the only other way to adjust the secondary light ray settings in this situation is to adjust the GI.
The other type of flicker that may be seen is what is known as “fireflies”. Fireflies appear as large dots of light and are most prominent in the specular and reflection passes.
As of VRay 3.0, it has been found that VRay Sphere Lights tend to cause fireflies in the reflection pass. Disabling “affect reflection” of the light does not solve the issue and neither does increasing the material’s reflection subdivision amount. If this issue occurs, the solution is to go to the Render Settings, under the VRay tab, expand Global Options - Advanced and enable Clamp Max Ray Intensity and set the value to 1.0.