Oh, Vue crashes during the prepass? Weird. What version, build number? Is it up to date? What's in the scene besides terrains and ecosystems? I never had this kind of crash...
To optimize the atmosphere and lighting, there are several things you can tone down:
If usinf AO or GI, you can put shadow smoothness back to zero, it is very rarely needed with these lighting models. If you're using GR best is to keep it as it.
Now the easyGI slider (quality boost slider) in the light tab can be set to -1 for AO and GI, and -0.5 for GR.
This will work just as good for outdoor renders, most of the times. But if you notice grain or noise in the render, then you'l need to up these settings.