Hi, Negative Lab Pro is intended to be used on negative film that hasn’t already been converted… so if you’ve used Vuescan to already convert the negatives into positive images, you cannot use Negative Lab Pro on them.
If you want to use Negative Lab Pro with Vuescan, check out the specific instructions for creating RAW DNGs with Vuescan. This will give you raw negative scans that Negative Lab Pro is expecting.
-Nate