Sounds like this has gone some circuitous way... :) If you have two vectors describing the cone their average is the the cone's centripetal axis.
Take the two vectors, linearly interpolate them to 0.5. Lets call that C Take your vector, measure its angle from the the one you got before (C) Measure the Angle between your two cone vectors, call it a Measure the angle between the vector you want to flag and C, if it's less than a/2 the vector is inside, if equal the vector is part of the cone's hull, if greater the vector is outside. If you want the cone to have a base you can also throw in some basic trig, but since this seems to be a vector outside range scenario, I imagine you will want the cone to be of infinite height. If you want it to be an "in potential reach" test, then it's not a cone anymore, as the base isn't flat but rather a section of a sphere, in which case before the above test also test that the length of your vector is shorter than the length of the bone, or it will always be out of reach.
