Dino - as we've discussed before, the VCs - while certainly beautiful - seem overpriced and finishing is not as fine as the APs (IMHO).
You also quite rightly point out that the 2121 used in the Extra Thin is the sibling of the VC 1120; weren't they both originally developed in a JLC-AP-VC joint venture? A lovely movement; great review on TimeZone about it?
As for VCs, I was very disappointed to see that the current VC use of the 1120 (their caliber for AP's 2120) used a rotor that was only part gold and the part with the Maltese Cross was in steel with a bland finish. When they had previously used an 1120, often times their entire rotor was gold, it was beautifully decorated and shaped like their Maltese Cross and was also beautifully engraved.
The movement you speak of was developed by JLC (of which AP owned 40% of up until a few years ago when it sold its interest). It is the only movement ever to be used by the Big 3 (PP, AP, and VC). It was never used in a JLC. Patek stopped using it in their Nautilus many years ago, but their original Nautilus with cal 28-255 is very collectible and valuable today. Also, unlike AP and VC which used calibers ending in some form of the number 20, PP's caliber based on the 920 did not end in a 20.
I think the Jules Audemars Ultra thin is a beautiful piece. I'm sure if you got one you would really enjoy it.