You could also try a 0 for frame spacing and make your crop land in the middle space. Make sure you click on "all frames" in your crop settings.
There is also a multi crop setting in Vuescan, but I've never used it. That might do what you want after you preview it.
Alternatively, buy an old computer that will run the software that was native to the scanner. I keep an old Powerbook around for the Nikon scanner when I want the color to be great without messing around with it. NikonScan works better than Vuescan for that, although for black and white Vuescan blows it away. Same with the old Canon scanner I have. Not as convenient though.
If all else fails, the best way to handle scanning a lot of frames at the same time is to set the preview resolution and the scan resolution the same. Then preview the whole area. When that is done, draw your crop over the first frame and hit SAVE. Then move the crop over the next frame and hit SAVE. Continue until you are done.
Hope that helps you.