The current VueScan release notes:... If VueScan uses the ICA driver, then that would also be an option. The web page doesn't say which driver it uses, only that Apple should auto-install it, which implies, but doesn't state that it is the ICA driver...
I should have some time to run some experiments on some "clean" OS instances in the next couple of days, but I suspect what happens in this mode is that VueScan is limited to the same set of abilities that Image Capture is. You are just getting the VueScan GUI.If your scanner isn't on this list, VueScan may still work with your scanner if you can find a WIA driver (Windows) or an Image Capture driver (Mac OS X) on the scanner manufacturer's web site.
I suspect the problem here is that Apple's Image Capture app, and the ImageKit (and Quartz) framework it is layered on top of, is too "lowest common denominator" and that non-mainstream stuff, like infrared lights/camera, multi-lens, etc. just aren't covered by the framework. Cameras and scanners were not covered by Apple's IOKit for drivers but by a different mechanism. Another contributing factor here is probably a "falling between the cracks" in the IOKit to DriverKit transition and this class of peripherals (and the scanner vendors wanting proprietary scanner apps as a product differentiation tool).
Brother and some other scanner makers are pointing folks at Apple's Image Capture app for 64-bit macOS 10.15 support. So, many scanners are getting up-to-date ICA drivers, but that is mainly to enable Image Capture, not the legacy apps. It would be smart for VueScan to ride that same wave, if they can tap into ImageKit foundations, too. (They'd pick up all the coverage that the standard bundled app got, too.)