Starting Out
First, you'll need to be sure and save both the RGB and Z-depth image from your Vue render. You may wish to save them in BMP format, as it is lossless.
Shown here are both the RGB and Z-depth renders as they come straight from Vue.
Note: The Z-depth image renders pixels close to the camera darker with those farther away lighter.
Copy Z-depth to alpha channel of RGB image
Go to Z-depth image and Select All --> COPY.
Then go to the RGB image, and choose the channels tab. Create a new alpha layer by clicking the new layer icon, then paste the Z-depth image.
Apply Lens Blur filter
Go back to the Layers tab and with the Background image selected, choose the Lens Blur filter from the Filter menu. You can then click anywhere on the image to 'set' the blur focal distance for your virtual camera.
In this image I clicked the vehicle in the foreground and so everything in front and behind becomes blurred and out of focus.
Adjusting the amount of blur
You can use the Radius slider to adjust the amount of blur. In this example, I clicked on the shoreline buildings, putting the foreground vehicle and the background mega-ring out of focus.
Final image
Here's the original image (top) and final image (bottom) rendered with DOF.