Why does this plane have vapor trails coming out of wings?
This are indeed the vortices a wing sheds where lift suddenly drops. That they occur at mid-span and not at the tips is due to flap deflection. They do indeed start at the outer edge of the Fowler flaps. @NilsNielsen explains well why you see a white line at the core of the vortex.

See this photo (source): This is the same thing happening. It is not specific to the 737-800, but to any aircraft with powerful flaps which end at mid-span to make space for the ailerons.