See this video of Birmingham strong crosswinds landings:
At 2.57, there is a 737-800 landing (screen shot below). One can clearly see some vapor (?) coming out of what seems to be two holes on both wings, from 3.03 to 3.10, were the plane lands.
This does not seems to be a 'classical" vapor trail, as asked in this question, as it clearly does not occur at the tip of the wing.
What is this? What explains this? Something specific to the 737-800?
Edit: While it is clear to me that this is a vapor trail, the question is more about why there is a vapor trail at this precise location of the wing. Usually, these appear at the wing tip, and this is clearly not the case here. Maybe something with the flaps, as suggested ? Thus, while the question itself is not exactly a dupe, this answer (and the associated pictures) give a clear explanation.