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. ;)