Well, with 2 pass unless you can "queue" things like in virtualdub, you'll just have to render the first pass somewhere, then come back and render the 2nd pass... It's fairly easy, you just set up x264vfw for "first pass" then click "render" or whatever, and then set it up for "second pass" in the same folder and click "render" again (overwriting the .avi or .mkv or .whatever). What "first pass" does is create a "rough draft" of the avi with weighing data and a ".stats" file (or something like that) that it uses during the 2nd pass to assign more bandwidth to more complex areas of the video. That's kinda the basics of it.
With vdub it's easy because you can queue the 2nd pass so it automatically does it without you starting it manually...
That's as easy as I can explain it :-D .