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" and "Scan" buttons do both the scanning and processing steps. The "Prev Mem" and "Scan Mem" buttons start from the raw CCD data in memory and only the processing step is done.