I suspect that the easiest indication that we will get is from the changes to SNRM (and presumably SNRM per band), actual sync speed, and attainable sync speed - which should improve in a variety of ways while the attenuation (and attenuation per band) stays the same.
However, if UPBO stays in place, we might find that some of the increase in SNRM is used to offset a decrease in the transmit power instead. But some of the studies on vectoring suggest that UPBO can be turned off with vectoring - so we might see power increases too.
I wonder if it is worth creating some extra graphs to help document this. I'm thinking of a scatter graph, that plots the attenuation along the X axis, and the attainable speed along the Y-axis. Each point plotted on the scatter would represent the pair of (attenuation, attainable) at one instant in time.
With one user, the accumulated points plotted would show the variation with crosstalk, followed, eventually by a jump as vectoring is introduced - and then variation after this point. However, all points would be on the same vertical line.
If the colour of the points was gradually adjusted over time (from light red to dark red, say), you'd get some idea of how the various points relate to time.
With multiple users results plotted onto one graph, you'd probably get a whole 2D space giving a clue as to the gains possible for vectoring over a variety of attenuations. Perhaps one day we could do the same with actual distance instead of attenuation.