I’m confused at your argument.
You can’t say he used the Masoretic, but you can say proto-Masoretic. The Masoretes did not yet exist. Yes, they used a certain list of texts we call Proto-Masoretic. It was not THE Masoretic because there are differences.
“Do you know of any instances where they used the LXX to help them decide how a certain vocalization should be done in the event of an otherwise ambiguous reading of a word?”
I’m not sure to whom you are referring here. The Masoretes? Why would the Masoretes use the LXX? The whole purpose of their textual selection was to enforce Jewish scripture that was clearly pointing away from Christianity. Why would the Masoretes use the LXX? They couldn’t really use the LXX for vocalization of Hebrew words because the LXX is in Greek. As I said, they used proto-Masoretic texts and some of these were the texts Jerome used.
Maybe I’m straining at a gnat here, but Jerome can’t use what hasn’t been written. It’s not a really big deal. I’m a just a sayin it was proto-Masoretic, not Masoretic. ;)
Look back. I agreed with you. The people involved in preparing the Masoretic text still faced a lot of problems in dealing with ancient Hebrew. There are instances of words that can mean different things depending on the implied vowels. One way of determining what they may have meant is to look at sources closer to the origin of those scriptures. The LXX was, at least, prepared by Jewish scholars who were a. from that region, b. over 7 centuries closer to the Hebrew in the source material, c. were pre-Christian. It would be an invaluable resource in helping to determine the meanings of ambiguous readings. And, yes, the Masoretes could have used the LXX to help determine vocalization of problematic texts. If there is a word in Hebrew that could mean A or B depending on which vowels are used and scholars much closer to the source text translated it as A rather than B, then you have an indication that at least this group of people considered that the proper set of vowels. In the absence of evidence to the contrary, you would cite that as a probable reading as well as give other readings as a possibility. Being a pre-Christian document, they could feel justified in doing this. After all, they claimed that they were trying to present the OT text as it was without reference to what it meant.