Hi Ray, I also have Vuescan on my computer but I only use it very occasionally for comparison purposes, so I can't speak with authority about it. Hence I don't understand how your SilverFast demo would be affected by Vuescan. Sounds perhaps like a driver conflict? I would suggest discussing this matter with SilverFast tech support.
As for scanner resolution and sharpness, the two of course are completely different concepts operating differently on the image. For starters, the effective resolution of your scanner has nothing to do with the self-rated resolution published by scanner manufacturers. It could well be that their CCDs are capable of rendering the stated resolution, but nothing else in these consumer-grade scanners usually can. Moat of them are optically more limited, or limited by other design and performance factors reducing effective resolution relative to rated resolution. The only way to evaluate this is to run resolution tests using a standard resolution target, as I have done on a number of scanners. In my experience, the Nikon 5000 comes closest to its rated resolution from amongst the Epson, Nikon, Minolta and Plustek models I've tested.
Sharpening is another matter altogether. This is purely a software function of altering edge contrast at the pixel level, changing the visual perception of acuity. Sharpness also very much depends on the content of the original media and its apparent graininess. Normally I do not perform any grain mitigation or sharpening in a scan, because these are non-reversible without rescanning the media. I send a completed scan to applications such as Neat Image or Topaz de-Noise or Noiseware or Noise Ninja (all good) for grain mitigation and Photokit Sharpener 2 for sharpening. This provides much more detailed control over these functions than available in any scanning software. Alternatively, the noise reduction (I know noise and grain are different) and sharpening tools in Lightroom have become so good that post-processing these scans in Lightroom is often all I need for these two purposes.
Bottom line: forget about the rated resolution of your scanner. Scan at the highest resolution your scanner allows if you think you'll need that resolution (usually for printing enlarged images) later in time, and then use the best techniques for grain mitigation and sharpening available (unless for some reason you wish to preserve the graininess rendered by the scanner/film combination).