Most foreign names, including the names of people and places, do not decline in Latin. Since there are a lot of foreign names in the Biblical text, this can sometimes be confusing! The declensions help to reinforce the meaning of the Latin, so it can be difficult to understand what a sentence means if the names do not have declensional endings. In the Commentary, I have tried to include a note about every foreign name when it is used in a case other than the nominative.
Sometimes the names do have a declension, or a partial declension. The name Moses, for example, has a declension which is based on Greek forms of the name. The name Abraham can appear as Abrahae in the genitive. The name Israhel can appear as Israhelem in the accusative.
The Hebrew plural form is found occasionally, as in the word Philisthim, which means "Philistines."
In addition, it can sometimes be hard to recognize the form of the name used in the Vulgate. Because of the sound differences between Hebrew and Latin, it was not easy to figure out how to write out the Hebrew names with the Latin alphabet! If you are confused about the names, go ahead and take a look at a modern English translation of the Bible to find the form of the name that you might be more familiar with.
Very often, the Hebrew names have a "meaning" (the way that a name like "Grace" in English has a word meaning, in addition to being used as a person's name). Since the meanings of these names are usually lost in translation, there can be some confusing passages in the Vulgate, where the meaning of the Hebrew name is important to understanding the meaning of the passage. If you are interested in learning more about the meanings of the Hebrew names, you can check any of the standard Bible commentaries and dictionaries, or at websites which provide lists of Hebrew names and their etymologies, like the Behind The Names webpage.