Scan your slide (it won't work well for negatives) and make output from VueScan in Device native color space.
Then (or before), scan the reference IT8 target (also without assigning any color space, just in device native one) and make icc profile with any (reasonable) tool (or maybe you already have scanner profile?).
Then in PS, ASSIGN this made icc profile to the scanned film.
Then CONVERT the color space to ProPhoto, AdobeRGB, sRGB or any other space for further processing.
In overall above processing you do not use any processing from VueScan, everything switched off (I only have Multi-sampling switched on and Lock exposure set to 1.5 - set by experimentation, but every scanner model is different, I use Reflecta ProScan 10T). If you do not have IT8 reference target, you need to play with all the sliders on COLOR tab, and save the image to any of above color spaces (eg. ProPhoto).