Wushen, do you have Photoshop?  If so, there is a very easy workaround.  Loading DEM data into Vue is a pain.  It's better to use the GeoTiff as an image map for your terrain in the terrain editor.

The problem is that the original GeoTiff is a 32 bit image that has to be converted to a 16 bit image.  I know that PS can do it.  Not too sure about other image editors though.

I did a couple of video tutorials about the process if you are interested.  Youtube compressed the heck out of them, so the quality isn't the best and there is no audio.  But it should give you an idea of how the process works.

Downloading and converting DEM data

Loading GeoTiff data into Vue 6

HTH,

Mike