How is the Vulgate rendering of Romans 5:12 explained (Roman Catholic perspective)? The Greek text (Textus Receptus) of Romans 5:12 reads:


  δια τουτο ωσπερ δι ενος ανθρωπου η αμαρτια εις τον κοσμον εισηλθεν και
  δια της αμαρτιας ο θανατος και ουτως εις παντας ανθρωπους ο θανατος
  διηλθεν εφ ω παντες ημαρτον


Which one fairly literal translation (The Orthodox New Testament Praxapostolos) renders: 


  Therefore, even as through one man sin entered into the world, and death through sin, and thus death passed to all men, on account of
  which all have sinned


But the Vulgate translates the final phrase ("on account of which all have sinned" - εφ ω παντες ημαρτον) as in quo omnes peccaverunt.  The Douay-Rheims translation of the Vulgate reads therefore:


  Wherefore as by one man sin entered into this world and by sin death: and so death passed upon all men, in whom all have sinned.


The implication is that somehow all have sinned in Adam - i.e. they inherit the guilt of Adam's sin.  Unless I am misunderstanding the apparatus, there is no variant in the Greek text that supports this wording.  John Chrysostom - a 4th century Greek Doctor of the Roman Catholic Church - does not seem to interpret the verse this way in his homily on the passage.  

My understanding is that the verse appeared this way in Latin in Augustine's De Peccato Originale, which also introduced the term "original sin" in the west, though perhaps that was not the first occurrence.

It seems that the Augustinian-Vulgate understanding of this verse has seeped into other English translations to various degrees.  While the King James is close to the literal translation above (And so death passed upon all men, for that all have sinned), other translations follow more closely the Latin meaning.  The ESV and RSV state, for example, ... and so death spread to all men because all sinned (i.e. cause and effect are transposed).

Is there some reasoned explanation for the apparent departure of the Vulgate from the Greek text of Romans 5:12?

(Perhaps this should be posted in BH instead, but since the passage of interest touches on theological differences between Christians I thought to post it here.)
In this case, in quo is a very literal translation of the Greek ἐφ' ᾧ, which is preposition ἐπῐ́ ruling the dative article ᾧ. In Latin, ἐπῐ́ translates to in. As Quintillian said, "Noster sermo articulos non desiderat", our language does not use articles, so the article has to become something else. Here it becomes a relative pronoun, and goes to ablative to satisfy Latin syntax.

So much for linguistics; what does ἐπῐ́ + dative mean?  According to Wiktionary, which sources Liddell &amp; Scott:


  (with dative)
  
  
  on, upon
  
  
    καθέζεται ἐπὶ θρόνῳ.‎ //
    kathézetai epì thrónōi. //
    He sits down on the throne.
  
  in
  at, near
  over
  in honor of
  against
  in addition to, over, besides
  (with duplication of head noun) after
  
  
    ὄγχνη ἐπ’ ὄγχνῃ γηράσκει‎ //
    ónkhnē ep’ ónkhnēi gēráskei //
    One pear after another ripens.
  
  in the power of
  according to
  (of conditions or circumstances) in, with
  (of time, never in proper Attic) at, on
  (of time) after
  (expressing a cause) on account of, for
  (expressing a purpose) for
  (of a condition) on
  for (i.e. in exchange for)
  (of a name) for
  in charge of
  


In all, 19 different senses. So which one of the 19 is right?  That depends on context, and different translators might validly pick different options here. The Praxapostolos you quoted picks sense 14, "(expressing a cause) on account of, for". The Latin in is closer to senses 1, 2 and 3.

Of course, the sentence must make sense theologically, so not all senses may be used, but if one does not start with a position where the sin of Adam cannot be the same original sin all inherit, then one can definitely come to this reading.