Why is the Concorde, and not other aircraft, getting this fog/condensation, over the wing, at *takeoff*?
You're correct that the condensation is due to low pressure, but the wing is not stalled. The highly swept delta wing of the Concorde created lift be what is called vortex lift. From my answer in that question:


  In the case of wings having sharp, highly swept leading edges like delta wings, the leading-edge separation vortex phenomenon occurs at subsonic speeds. However, the separation does not destroy the lift as in the case of low sweep wings; instead, it forms two vortices which are (nearly) parallel to the wing edges.


The centre of the vortex is a low pressure region, where condensation forms. In this figure, you can see the vortex formation in a model of Concorde.



Flow around Concorde in a landing Configuration by Henri Werlé, ONERA, image from efluids.com

A side view of the same thing:




Flow around Concorde in a landing Configuration* by Henri Werlé, ONERA, image from efluids.com


So, basically, the aircraft is still producing lift; only the mechanism is different. 

This kind of lift production is quite common in case of highly swept wings. The vortex formation is also found in a number of other cases like chines, leading edge extensions (like this F-18), etc.



Flow visualization over a 1/48-scale model of an F-18 aircraft inside a water tunnel at the Flow Visualization Facility from NASA's Dryden Flight Research Center. Image from wikimedia commons