I am following a tutorial where I've created a single vert object and am now trying to a subset of the vertices. However, I only two vertices are selectable as shown below the tutorial's example. Perhaps, I have accidentally hidden the vertices in the 3d Edit mode, but I couldn't find where to toggle that feature. Thanks in advance!
If you're using the skin modifier on vertices that you keep on extruding, the vertices are inside of the mesh, not on the mesh surface. You'll need to enable xray.
If you've hidden any vertices, in edit mode, try a to select all vertices, then alt + h to unhide any hidden vertices.