For Vista you will need DotTwain 2.1 or higher.
I've seen similar issues with Brother scanners and it was a bug in the driver. Basically what was happening is that the pages were being scanned into the scanner's internal buffer before returning to DotTwain; and it would give us the last page it scanned.
Try setting the AutoScan property to false. That should turn off the internal buffer, although I can't be sure that will work with a WIA driver.
Glenn Chittenden Jr.
Atalasoft Development Team