First off, we don't know what manuscript Jerome was writing from or where he learned his Hebrew or how he viewed the sanctity of the translation process. Jerome did not, for example, have the Masoretic text that we do today. He was writing about 600 years before that text was compiled (MT is from roughly 1000AD). Anti-semitism was pretty big throughout these eras, so Jerome worked in relative secrecy with Jewish Hebrew scholars.
We know, for example, that right away in Genesis 3:15, Jerome translates a masculine pronoun into feminine because he saw the phrase about the serpent and the woman as a prefiguration of Mary/Christ. So "mary" (she) would bruise his heel. He felt justified in these kind of translations. We have no extant manuscripts that I am aware of that have this pronoun that were not derived from Jerome's work. We are actually not even sure if Jerome made this error or if it was a later scribal correction to his work. Jerome was a stickler for the accuracy of the Hebrew and frequently got into academic/theological battles with his fellow church leaders on his translation.
Here is Jerome to Augustine quipping over the translation of a word related to a certain plant described in the Book of Jonah:
But if your Jews said, either through malice or ignorance, as you yourself suggest, that the word is in the Hebrew text which is found in the Greek and Latin versions, it is evident that they were either unacquainted with Hebrew, or have been pleased to say what was not true, in order to make sport of the gourd-planters.
That's what we call a sick fifth century burn!
That being said, Jerome is trying to translate an idiom in Hebrew. This is likely an ancient idiom that even the Jews he was secretly working with did not understand fully.
The word for "hip" is yarek and is certainly not "hip"... This is the place that Jacob gets touched when he is wrestling with the shadowy figure by the river in Genesis 32. Jacob (father of hebrew tribes) is renamed to Israel (who is the bride of God for the Prophets) and there is something distinctly contrasted as feminine in Jacob compared to his ruddy/hairy/masculine brother Esau in Genesis 25:27. Jacob is cooking soup and has to dress up as a masculine hairy male to fool Isaac.
Close to Samson in Judges 8:30, Gideon's many offspring "came out of his yarek" and in Exodus 37:17, the "shaft" of the menorah in the tabernacle is called the "yarek."
There are definitely phalic and genetive (e.g. offspring) elements to this word when it is used. In Judges 3, Ehud straps his sword to his right yarek, and the whole episode is full of phalic imagery, of the double mouthed sword the length of his arm, strapped in his crotch region and then that it was buried in Eglon's womb in a kind of male-male rape image. The idea that this is "hip" is pretty puritanical or chastity culture based, which was certainly an aspect of Christianity if Jerome's time, but obviously not for the author of Judges which reads more like Game of Thrones.
That being said, there is a sense in this passage of some sort of idiomatic description of Samson's slaughter of the philistines. Like "cutting them from crotch to hip" in a sort of emasculating move. The idea that they were "running away" does not match the attitude of the Philistines in the Samson narrative.
There is also a repeated "k" sound in the verse which have that kind of onomatopoeic "cutting" sound.. The verse sounds like "way-yeK owtam sowQ al yareK maK-Kah" meaning "he cut them down leg on crotch with a slaughter." This sound is also present in words like carat meaning cutting as in cutting a covenant. Hebrew is certainly an onomatopoeic language. Maybe the poet was picking these words because they were loosely related to the anatomy and carried the sound of cutting through the story?
But the bottom line is that we don't know what this idiom means, and it is clearly an idiom describing the way that Samson slaughtered the men. Did Samson slice them in half up their middle? Did he cut them across the waist so they were a severed torso and then just a crotch over legs? Something else entirely?
Jerome was clearly trying his best to translate what was in the Hebrew.
The Septuagint version was roughly 500-700 year earlier than Jerome and really doesn't help much either (and he had a copy of this available for his translation). The greek words κνήμην and μηρόν (leg on shin?) are relatively obscure too according to the lexicons I have explored. It is also unclear that the Septuagint, from the apex of the Hellenistic period, is a better source either. Clearly the authors were up to their yareks in Alexandrian greek culture.
Clear as Mud? It's an idiom. Meaning is unclear. It'd be like trying to pick the right words to translate "he got bent out of shape." Certainly can be translated, but there is a meaning not conveyed by the words alone that can't be written unless that meaning is added on with the text.