That would be to hold down the "v" key on your keyboard and MMB klick a vertex.
If this doesn't work, klick in the middle of the manipulator, to light the small yellow square, and then try again.
If you (while holding down "v") first MMB klick one of the manipulator's arrows, and drag your cursor to a vertex, you get to snap in the axis direction.
I do not support Bush! Nor Saddam! Long live UN!