When one vertex obscures another in the current viewport, there are two techniques I have found useful for selecting the one behind. One is to modify the view slightly, for example, shifing the viewport a bit to one side, so that both vertices are visible. Then I can select the one I want, and after making the selection return the viewport to the original orientation, if desired. Another technique that sometimes works is to select a second time. Often the first selection event selects one of the vertices, and a second selection at the same place toggles to select the other.