As it turns out, for some mind bending reason, a number of video editing apps do not output the correct Mp4 file header to allow the video to begin playing once the first few seconds have been buffered.  I cannot fathom why this is.  It’s certainly not the case with WMV (Windows Media) or MOV (Quicktime) files.  In fact, Vegas has a checkbox for  both file types that correctly encodes the file for streaming.

I finally ran across this great little app called “MP4 Faststart”, and it actually corrects the problem.  It simply processes the file, and updates the header to allow a progressive download as the video plays.  Amazingly, it’s free.  Well actually, it’s donationware, and I think you’ll find it’s worthy of a few bucks to the developer, as it will save much head banging.