There are two different things VueScan does - scanning an image and processing the raw data from the scanner to produce a color-corrected image. These two things are done in steps, and it's useful to understand each of the things that are done when scanning and when processing.
The "Preview|Device" and "Scan|Device" commands do both the scanning and processing steps. The "Preview|Memory" and "Scan|Memory" commands start from the raw CCD data in memory and only the processing step is done.