I'm not really sure if it's DOF or something else, but this scene isn't looking quite there.  Maybe the texture on the mountains?  It looks like they're too close? 

I'm using Global Illumination, some fog and haze.  I'm trying to make it look as realistic as possible, any suggestions?  Help please!!

Thanks in advance!

Dof? I don't see any dof effect here, but I do think it lacks some haze. Hard to see if you are using a spectral atmosphere, but if so, in the sky fog & haze tab, change the aerial perspective parameter to 30 ( manual input). This parameter "shrinks" the skydome to fit the scale of your scene. The higher the value, the smaller the skydome, so the distant haze becomes more visible.
If non spectral, add more haze.

In the real world, with a photograph, you would not have out of focus areas in such a shot. Adding them in Vue will not help you get your scene more realistic.
Maybe try to get the camera further back and use the Vue equivalent of a tele lens (300mm+)

Thanks so much guys, I'll give your suggestions a shot. :)

Offrenche ... not sure that's right - if you focused on the foreground cactus with a wide depth of fieldshort focal length, the background would be out of focus.

Likewise if you focused on the background with a large depth of fieldshort focal length, the cactus would be out of focus.