What suggestions do the defenders of the Septuagint and the Vulgate have for the alleged insertion of "rock" into the text? I can not see why the compilers of the Masoretic text would have done so. Also, knowing the extreme care that they used when handling the Word of God, to add a word that wasn't there at least in some text that they considered reliable would seem unthinkable.
At the end of the article are all the ancient Hebrew text sources I could find. There is a copy of Deu 32 from cave 4 at Qumran (Dead Sea Scrolls) but it does not have the verses we need.
Nevertheless, as you say, copying the Holy Scriptures was a very solemn thing to the Jews over millenia. And especially so when one was copying a Name of God
Instead of "God is the Rock" the Septuagint says "As for God, " No defense has been offered for the Vulgate which omits it altogether.
This is very important to me because the name "God is the Rock" is being specially announced in Deu 32:1-3. Moreover, as 1000 Silverlings notes - it is in the Song of Moses (which will be sung in heaven - Rev 15:2-4.)