Hi
I had a look at one of your unplayable files..Marijke_Shoot_3.mpeg4 , an H264 MPEG4 1920 x 1080 25fps clip. This loaded into VP but did not produce a thumbnail in the clip bin. Dropped into the timeline It did display a thumbnail but the clip did not play in either the clip preview window or the sequence preview window. It just showed a thumbnail whilst the soundtrack played. I then checked the VP cache and found there wasn't a full xxxx-xxx-x.video .cache file which suggests VP could not properly index it. It may be a corrupt file (files)
However I converted this video to an mpeg2 video with the same resolution and this played perfectly in VP. (I used AVC)
Out of curiosity I then converted it into an MPEG4 file (even though it was already an MPEG4) and this also played perfectly. NCH may be able to see where the glitch is but I would suggest you simply convert all the unplayable files "back" into MPEG4.(or use MPEG2 which looks almost as good.) If these are files you originally exported with VP I would suggest you export using the basic MPEG4 settings and not H264. under the encoder options. Then see if you have the same problem.
Nat