Sorry for the delay, I've been away.
Yes, the JDSU will happily show an 80/20 sync.
Detecting a partial/rectified split pair can be tricky, the end to end PQ (pair quality) test *may* show low AC balance, 60db or greater being ideal, but over 55db for sure.
With the JDSU in manual sync on the VDSL, see if you can peer over the engineers shoulder, note down the theoretical max sync, the attenuation and the snr. Oh and of course the current sync rates.