Question 1061268
Nothing you can do; vertical line means no value is greater or less than "y"; is this question about an optimization problem, like "linear optimization" or "linear programming"?  If this is your case, then the vertical line is probably just part of the boundary description for the region of the solutions you are looking for; and you will be most interested in the intersection points of the vertices defining the region.  The fact that a slope of one of the lines defining the region does not exist will be no obstacle in finding all the vertices for the region of points to check.