Thanks everyone for your valuable advice,
So with an incorrect QNH set the VSD will show the non ILS approach slope back from the touchdown point on the runway (magenta line).
The VSD aircraft symbol, if in VNAV path, with be positioned on the this magenta line and showing this vertical track leading to the runway touch down point on the VSD runway symbol.
The "Aircraft symbol" and "Runway Symbol" will both APPEAR as being at the correct elevation in relation to the VSD altimeter tape at left side of VSD.
However the terrain picture at the bottom of VSD will not be at the correct elevation in relation to the runway symbol, due to this terrain information coming from the EGPWS (TAAS) which receives radio altimeter inputs when the aircraft is below 2500ft.
So with an incorrectly set QNH, it will only become apparent, on the VSD, when the runway symbol appears in view on the VSD, and this terrain will show above or below the runway symbol.
A altimeter error, creating aircraft actually 250ft low, would show the green terrain line about 3mm above the runway symbol...
Thank you.