Depth of Field on different Vcams: Unity 2020

Assuming you're using HDRP or URP, then you can add a CinemachineVolumeSettings extension to the vcams whose DOFs you want to modify. Add the PostProcessing/DepthOfField override to the custom profile in the extension and set the DoF there.

It's a good idea also to have a default global volume with the default DoF setting, so that the blends work well. They don't always if there is no default DoF setting.

 

Yes. You don't have to add it to the main camera, you can add it anywhere. If it's global and active, the renderer will pick it up.

Volumes in general are not associated with specific cameras. They will apply to any camera. Vcam volumes are attached to the vcams only to blend in the effects along with the vcams. Under the hood, they are just creating dynamic global volumes and blending in their weights.