To Find the Focus Point, download the Plugin script "Advanced Camera Tools"
1. Go to Advanced Camera Tool (see image below)
2. Create Camera
3. Name the camera and click ok
4. Lock the camera
Now you will have the camera icon in SketchUP (see the image below). Now measure the distance from the focus point to the lens of the camera icon. Here, the measurement is 12 inches. THIS IS THE NUMBER I ENTERED IN OVERRIDE FOCAL DISTANCE IN THE VRAY PHYSICAL CAMERA.
Note: Vray units in DOF is measured in inches. If you are using meters, be sure to convert this in inches.
Finally here is the comparison between the Sketchup view and the rendered seting.