Vacheron much less popular in the Western world but a significant brand in China. In China this is THE brand to own along with PP..... but due to price point this is more popular.
Nothing in their line up excitws me that much..... most interesting is their Lemania Chrono Perpetual, Malte Chrono. But they are not unique and they are not the "best" out there.... their PP equivalents are often seen as superior.
Is this the difference? The lack of a unique and/or superior quality? going through your list one by one:
The AP's you mention all have a unique styling aesthetically,
The Lange 1815 Moonphase is incredibly accurate m/p I think
PP - is just PP and their complicated models are very rare, along with histry etc
Panerai is another uniquely style watch and older wtches are obviously rare
Rolex Daytona..... never really understand, but this is the ultimate mass market luxury watch that everybody in the world knows
FPJ Vaga...... just 1 freaky auction selling at 90k?
Undiscovered Gems?
Dato - Readily available and with all the price rises over the last few years by everyone, no longer seems "expensive" in relative sense!
Resonance, still selling at big discount. Definitely not underappreciated, but "cheap" for such a rare/unique watch
Choppard LUC - Most models are fantastic. 1.96, 9.96, Perpetual, Strike One, new Chrono.
Blancpain does offer great value new and old. Split second is affordable, Perpetual with adjusters under lugs is cool, monopusher Chrono, the list goes on with their useufl complications done in their own special way.
Ulysse Nardin - The Perpetual GMT movement. So unique and if found in a PP would be attched with huge price tag. cheap new and used
Glashutte Pano - Germany's "second" brand but their double swan-neck in thie range seen from the back is very very nice! The Panograph is a most unique and interested chrono.
Clearly, I think that most if not all watches that sell significantly above list are rare and/or unique, by no means does it mean that all rare/unique watches sell above retail!!!!! This is the missing X-Factor. The Undefinable