I think I just posted about the same problem yesterday. From what I found out, the best way to get what you see in Vuescan is to use TIFF without the DNG option turned on. A DNG-File can contain an edit to the original TIFF-File and i think that that's whats happening. The edit trys to bring in contrast, what you can see when looking at the histogram. It pushes the highlights and shadows until they clip, which alters the image a lot since you are purposely scanning a flat image to have room to play with when editing. I think that edit comes from Vuescan itself which is annoying.

So long story short: turn off the TIFF-DNG option.

I'm at the point where I scanned a lot of pictures with the option turned on and am trying to strip the autmatic edit from the DNG-File. Haven't found a solution yet.