Thanks you, Jan. Very informative. I just noticed that when I leaned into the heel gap when standing, the sides puckered. My foot has normal to high arches, so it's not bad on my shoes. Strangely enough, the supposed voluminous P last does it less than the updated P2. P is tighter than P2 for meI hope I am not misunderstanding the above. If I am, my apologies.
As a flat footer I can confirm that gaping improves over time with Vass, and your foot will get more support. But the heel leveling is most likely a factor only when standing (entire sole on the ground) as opposed to walking.
When walking, other factors are more important, at least in my case. Vass lasts expect yout to have a large foot volume, which is typically incompatible with flat feet. (Sadly, their derbies have unnecessary wide wings to mitigate this). If your gaping is attributed to a low volume, heel wear will not make much difference. Most of the gaping happens when landing on the back of your heel at any rate, when you shift balance from one leg to the other. This will happen even when the rear of the heel is worn out.
Btw. if this is a construction flaw then VASS appears to be selling only poorly constructed shoes.