If you have access to the public family trees on Ancestry.com (which is where this illustration came from) you can easily figure out someone's mitochondrial line.
1. Once you are in the person's chart change the view from Family view to Pedigree view.
2. The bottom line of ancestors is the individual's mitochondrial DNA line.
It this instance, you can see my mitochondrial line goes from me (Teresa L. Snyder) to my mom, to my maternal grandmother (Katheryne C. Lynch), to her mother, my great grandmother (Laura Jane Feasel), to her mother, my gg grandmother (Elizabeth H. Armstrong) and not on the screen, Elizabeth's mother, my ggg grandmother (Leah M. Shupe). This is as far as my research has taken me, but obviously, this line would go as far back as the beginning of my maternal line.
If you change the individual to my mother, you can see that her mtDNA line and mine are exactly the same (except now you can see my ggg grandmother, Leah Mary Shupe.)
But look what happens when I change the main individual to my dad.
My father's mtDNA goes through his mother (Anna Martha Schrader), to his maternal grandmother (Emma Augusta Gleffe), to her mother, dad's great grandmother (Pauline Gliffe), to her mother, dad's gg grandmother (Wilhelmine Bayack).
In the case of the Vinton County Group, we are looking for individual's whose mtDNA ancestral line goes back to the Napper, Thacker and Dorton women of the clan. As you can see by this, neither my father or I have the right mtDNA line to be tested.