John, thanks for the info, much appriciated.
PTgui 9.1.2 works great, no more errors with VP correction + faster then previous versions.
Below a 100% crop comparison of the nadir (of equirectangulars) of a hand held pole pano without and with VP correction (after run 1 VP correction was switched on, then optimized with the same CP's and output as run 2 ).
The relatively small improvement of just 20% less CP errors by using VP correction is obviously enough to help to get a better result with Smartblend (my prefered blender for handheld pole panos).
I see this improvement often, hence my use of VP correction for handheld pole panos.
I think VP is too technical. It is effectively camera position. So if your camera position is at the right point (NPP), then further optimization will not help. If not, it can do some transformation to bring it closer to the right point, just like in your handheld panos.
Ha Ha Wim
This is no difference to take a nadir. Actually this is how I take the nadir. Of course you can use a VP on the edge of a fisheye if you mask the edge as a separate flat area.
However I would recommend you to shoot at -10 for best results.