The Vray sphere fade volume node lets you render everything inside a spherical volume and disregard the rest. To set this up first enable the “use environment volume” in the Vray > environment tab in the render settings. Then map a volumetric > Vray sphere fade volume  to the input map. Then create create > V-Ray > VRaySpherFade volume. Control the bg color, sphere radius etc through the attributes on the VRayspherFade node.