I've bought the Vuescan bible and bought the lastest Vuescan at the same time.
I'm trying to scan a few cloud B&W negatives from my 4x5 camera.
When I do the preview, Vuescan will automatically compress my range to 0 - 255.
The 0 -255 range is a result from your choice of 'Input' tab parameters. When you set the 'Bits per pixel' to 8 bit gray, then the resulting output data from the scanner wil be a single gray channel file with 8-bit data. VueScan will usually acquire the data directly from the scanner as linear gamma data, which will then be adjusted according to the settings of the 'Color' tab, such as clipping points, brightness, and gamma.
In the case of an 8-bit per channel data input setting, it is possible (depending on the scanner firmware) that the scan data (which is originally in 12 to 16-bit per channel inside the scanner) is already leaving the scanner as gamma adjusted 8-bit data with the use of a a lookup table in the scanner's firmware. Doing the gamma adjustment in 12 to 16-bit and then convert to 8-bit is to avoid loss of precision when the linear 8-bit data would require significant gamma adjustment later in software. VueScan is clever enough to choose the method that produces the best quality.
I've played around in [input] & [color] to try to get a flat look which would be more realistic to the original scene.
But I still can't fix the shadow part, my histogram looks very broad rather than so narrow.
I'm not exactly sure what you want to achieve by 'fixing the shadow part', but you should probably make sure that on the 'Color' tab your Black point clipping is at 0%, and your white point clipping is at a setting that just avoids clipping if you want contrast in the clouds, or the White clipping point is set at 100% (=1) if you want to keep a more moderate cloud contrast.
Is there anyway turn the black into some kind of dark gray during the scanning procedure in Vuescan?
The 'Brightness' controls are just gamma adjustments, so you may need to use a dedicated image editor instead of a Scanner utility to position the darker end of the histogram at a default higher level.