Hello,
Whatever I do LR asks me to select a “darker neutral area” while I try to do the whit balance on orange mask.
I use VueScan in RAW with every parameters as told on the guide. My scanner is a Nikon CoolScan LS-8000.
Thank you for reading me 
UPing since it’s been a while
I run into the same issue when I attempt to use Lightroom’s color picker on film borders. This happens with Raw DNG files acquired from Vuescan, as well as with TIF files acquired from my drum scanner.
I’ve read elsewhere that people have had luck using the “auto” white balance feature in Lightroom, but I personally don’t prefer the results I get after auto-white balancing. As a result, I haven’t been doing any white balancing at all. Instead, I just take the file straight from the scanner, crop the image borders (or leave the image borders and use a large enough buffer), and run it through NLP. It seems like NLP does a really good job of removing the orange film mask on its own.
I’ve tried about every permutation I can think of with regard to white balance, border size, TIF Scan Prep utility, etc., and I’ve found that, in general, I prefer the results I get with the simple approach described above.
I have the same problem and my ‘solution’ for now seems to use the WB picker on the black area of the negative holder. Still have to move around a little to get the values close to each other but the results are better manageable for a complete roll of film than letting NLP remove the the mask.
As far as I know, the WB picker on the negative scan is to even out the values and remove the color cast, which also (should) work(s) on any other neutral value, like the black of the negative holder. I don’t crop before I convert, I use the buffer.
I might be doing it wrong or am misinformed…
This seems to be a duplicate of: "White balance cannot be set" how to get a darker film border for setting white balance?
Have a look there