Nothing wrong with vuescan except that it's a complete bitch to use; it will only clip if you do something wrong, which is admittedly too easy. However it's 10x better than any bundled scan software I've ever seen.
Select "Lock Image Colour" once you've set the film base colour and re-previewed, that will give you the separate sliders. I find you want the B&W points for blue to be a bit further right than for R/G when doing Ektar. I also find it much harder to get accurate/neutral colour from Ektar than any of the Portras.