Some of CSGO's files as shipped are, for lack of a better word, corrupted.
However, there is a solution for this: delete the <filename>.vtx file, but make sure you leave <filename>.dx90.vtx in place.
That should let you import them.
While I'm at it, quite a lot of them also have the wrong internal filename that overlaps with another one, and since I'm using that to name them, there'll be "missing" and "wrong" models because of this. The little checkbox to the right forces HammUEr to use the filename instead of the internal one, and should take care of most of them problems. (You might have to re-import the "real" name ones if some of them got overwritten anyway)