When and where?
Scholarly consensus since Seeligmann (I. L. Seeligmann 1948, 75) has been that Isaiah was translated into Greek in Alexandria around 140 BCE (R. Troxel 2008, 24), although dates as early as 270 BCE have been proposed (Margoliouth 1900, 4–7) Van der Kooij has argued that the translation of Isaiah was by Jewish scholars who fled to Egypt in the first half of the second century B.C.E. (Arie van der Kooij 1981, 50–65); see also his “Servant of the Lord” (A. van der Kooij 1997, 395) and “A Short Commentary”(Arie van der Kooij 1982a, 71), along with (Ziegler 1934, 31–46) (I. L. Seeligmann 1948, 39–42), who argue for a consistent translator(s) for the whole of Isaiah. The basis for this identification is the Egyptian vocabulary in Greek Isaiah. Judging by the kinds of mistakes made by the translator (that should have been caught by a proofreader), and the consistency of the translation, I would argue that the translation was made by an individual rather than a group.
At that time, the vowel points developed by the Masoretes in the sixth century CE were of course not recorded in the translator’s Vorlage. It has been suggested that the translator may have followed a different tradition (Ottley 1906, 7), but I argue that the translator did not make use of the resources of any reading community.