4. Anti-aliasing filter
This is another feature that is not strictly limited only to the Progressive Rendering option. It is key to image sampling though, so it's well worth covering. It enables you to specify whether you want to sharpen or blur your image based on a specific algorithm. You can obviously do this in post if you would like, but V-Ray gives you the option, too. Avoid using the Progressive sampler with sharpening image filters (Catmull-Rom, Mitchell-Netravali) as this may slow down the rendering and additional image samples will be required to resolve sharpening filters properly.
Use the drop-down to specify the anti-aliasing filter and then the size variable to set how much it affects the image
5. Min and max subdivisions
This is the key part of the progressive rendering option, so listen up! The image sampling tool is responsible for refining your rendering to make sure all of your geometry is as accurately represented on the screen as possible. By giving you a minimum and maximum subdivisions, V-Ray is essentially setting up an adaptive solution. It starts at the minimum number of subdivisions and uses that amount if it believes it to be sufficient. It then progressively moves up to the maximum subdivisions if it determines they are required. More on how it determines this in the next step...
Here you can see two rendered images: the left one shows a great result with 100 subdivisions and the right one is the result of only 2 subdivisions
6. Noise threshold
This setting is your desired noise level in the image. The lower you make this number ? assuming the max render time is high enough ? the less noise that there will be in the final image. This defaults to 0, which means that the render will sample uniformly until either the max subdivisions value is reached or the Render time limit is reached. More on that in the next step...
Here you see two more rendered images: the left one shows a great result with 0 noise and the right image shows a terrible result with a noise level of 1,000
7. Max render time
This is the maximum amount of time in minutes that the render will be allowed to run for. The majority of the time you will probably just want to leave the render until you are happy with the quality, but if you want to set a time limit then it's good that you can do that. It's important to note though that this time does not include any global illumination pre-passes, like the light cache or irradiance map. If you want to ensure that the render carries on until the maximum subdivisions are reached, then you will need to set this value to 0.
The render time seems to defeat the purpose of making it progressive but it's good to have the option all the same