Okay, so you're pretty happy about your Scanwit 2740S and VueScan. You scan lots of slides that usually are taken in broad daylight. Then it you take a picture of something when it's dark, like these hyena cubs taken in Kruger Park:

One way of improving this scan is changing the whitepoint and blackpoint levels in VueScan. You find this at the color tab. I've noticed that with for example whitepoint you can adjust the overexposure of white in the picture as it where. See the examples below

bp=0 bp=.3 bp=.6 bp=1

As you can see, increasing the blackpoint value a little has a big effect on the hazyness in this picture. To see the effect of changing the whitepoint value, notice how the brightness of the hyenas ears changes.