Congratulations Klas, you've found one of the ASLRB's black holes. There simply doesn't exist any rule telling us how wall TEM applies when the LOS is drawn to a vertex or hexside :nuts:
The only rule we have is B9.3, saying "Fire traced through a wall/hedge hexside or hexspine may be subject to a TEM for that wall/hedge if the target is in the Location formed by that hexside/hexspine.".
Note that this requires the LOS to go through, i.e. into and out of the wall, not only into. The problem is that when a unit is in bypass, the LOS is drawn to a vertex, and therefore only into the wall, never out of it, and thus never through it.
So, if we were to follow the existing rules, a wall belonging to the target vertex could never provide TEM. This is not how it works though, since the A4.34 example shows several situations where such a wall provides TEM. So there must be a rule for this, except that it isn't in the rulebook. The example tells us enough to give a clear answer for your question though: The M12/N12 wall provides TEM.
If I were to write a "vertex LOS and wall TEM" rule, I would suggest that the area from where the LOS would be obstructed by the wall, would be the area bordered by (and including):
1) The alternate hexgrain that the wall hexside is part of, and
2) The alternate hexgrain going through the target hex' center dot and the target vertex.
Fire from anywhere between those two alternate hex grain borders should receive wall TEM, fire from outside those two would not.
In the provided example, the two alternate hexgrains would be:
1) One starting at the M12/N12/M13 vertex, and going through N11's center dot.
2) Another going from M12's center through N13's center.
So The M12/N12 wall would provide TEM to the M12/N12/M13 vertex, from N11-13 and O10-15, but not from N10, N14+ or M13+.