Re-thinking what is needed is not necessarily the point receiving the incidence angle but instead assigning the polyline layer an attribute with the value of the difference between adjacent vector angles. So, in the example above, without doing the math it appears that the incidence angle would be nearly 90 degrees. (The angle measurements appear to be from the X-axis.) Once that value is computed a query showing which vectors have an incidence angle greater than or equal to 30 degrees could be made.
Any thoughts about the math needed to compute this value would be appreciated. High school geometry re-visited!