Can you play them back on the camera itself? If no, then the data is corrupted.
Secondly, have you tried a different card? If no problems surface with a different card (I don't have experience with Lexar, only Sandisk and Transcend), then the card might have issues.
Have you tried ingesting it in an NLE? If this works, then it might be a file-writing problem and you might want to get the camera checked.
Also, have you tried Quicktime? If this works, then I know there's a Nikon-Apple conspiracy...just kidding.
If VLC+QT+NLE can't play it back, then I usually give up hope.