The question of reliability for the LXX and Masoretic text is interesting. I'd have to say that the LXX versions are good for learning the Greek tradition of the text and the Masoretic text is good for getting started on a Hebrew reconstruction. IMHO, they should be used together, and the LXX can be used to "backtranslate" into Hebrew for comparison to the Masoretic text.
As for the Vulgate.... many Greek, Hebrew, and Aramaic texts that Jerome used - if I remember correctly - are no longer available today. Where the Latin deviates from the texts that we now have are noted in modern critical editions of Greek (and I believe) some Hebrew texts. That way the scholar can go to the Vulgate and backtranslate into Greek (or Hebrew, etc) to reconstruct the nature of the missing text and compare it to those which are still avialable.
