You are outputting DNG's in Vuescan, which isn't a very good idea.
Been there, done that, lots of problems. http://www.luminous-landscape.com/forum/index.php?topic=52578.0
Basically it boils down to the fact that Vuescan DNG's are not proper raws, they are rendered untagged files. Preferably you want to process rendered and tagged files in LR3 rather than untagged rendered files.
A remedy is outputting your scans in Vuescan as 16 bit Tiffs and use Prophoto RGB as outputprofile.
Hope this helps.