In the comment section of the previous post (on "Asceticism") I mentioned the so-called "Umlauts" in Codex Vaticanus. Wieland Willker has set up a very valuable section on his website that deals with the Umlauts.
There he discusses a number of issues, e.g., the dating of the Umlauts over which there is disagreement among scholars. Leaving aside the "material" side of the problem (that has to do with the nature of the ink, etc), I assume that a complete survey of the textual support for each variant (at least at the un-ambiguous places where we can identify a particular textual variant) could bring us a step closer to the question of the nature and,in extension the dating of the Umlauts even if I realize that the evidence may be open to different interpretations.
In any case, besides Payne, Canart, Niccum and Miller, I know that one scholar in America is working on a thesis on the subject. Moreover, I know that C.M. Martini is working on a new study of Codex Vaticanus, but I don't know to what extent it will deal with the Umlauts. Are there any others out there?