A320 FO BECOMES CONFUSED DURING VECTORS TO RWY 19R AT LAS; DSNDING BEFORE CLRED FOR THE VISUAL AND NOT COMPREHENDING THAT TFC IS ON R DOWNWIND FOR RWY 19L. A LATE DSCNT CLRNC RESULTS IN AN UNSTABILIZED APCH AND A GAR.

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WE WERE ON A DIRECT HEADING FROM GRNPA ARR TO LAS VOR. AT 5100 FT CROSSING THE HILLS NE THE ARPT; WE WERE CLRED TO A HDG OF 270 DEGS TOWARDS THE STRATOSPHERE. AT THIS POINT I HEARD APCH TELL US THERE WAS AN ACR ON L DOWNWIND TO RWY 19L; AND WE WERE CLRED VISUAL. SOMEWHERE IN THIS; WE WERE CHANGED TO RWY 19R; WHICH MADE SENSE TO ME WITH ACR GOING TO RWY 19L. CROSSING OVER THE HILLS; AS THE CAPT WAS CHANGING THE BOX TO RWY 19R; I STARTED MY DESCENT; AND THE ALT ALERT WENT OFF. I SHALLOWED THE DESCENT; AND ASKED HIM TO 'ROLL IT UP;' AND HIS RESPONSE WAS WE WERE NOT CLRED DOWN YET. SAME TIME APCH WAS ASKING US TO CLB BACK UP DUE TO ACR TFC OFF THE NOSE AT 4000 FT. DURING THIS HAIRBALL IS WHEN I FOUND OUT THAT ACR WAS ON A R DOWNWIND (W OF THE ARPT) AND CLRED TO RWY 19L. WE WERE KEPT ABOVE AND BEHIND THEM FOR RWY 19R. IN A SHORT TIME; THE CONFLICTS WERE RESOLVED; WE SWITCHED TO TWR; AND WERE REQUIRED TO MAKE A HIGH ENERGY DESCENT TO GET ON PROFILE. UNABLE TO STABILIZE THE APCH BY 500 FT; I ELECTED TO GAR. WE WERE VECTORED FOR A R DOWNWIND BASE FOR RWY 7R AND UNEVENTFUL LNDG. TOWER/APCH QUERIED AS TO WHY THE GAR. WE TOLD THEM KEPT TOO HIGH TO ALLOW TFC TO CROSS UNDER FOR LNDG. MY LESSONS LEARNED. I WILL OBVIOUSLY BE MORE ATTENTIVE TO THE CLRNCS IN THE FUTURE; ESPECIALLY WHEN ARPTS ARE RUNNING AN IRREGULAR PATTERN THAT RARELY HAPPENS. THE COMS WERE VERY BUSY; AND THIS LED TO MY MISINTERPRETATION. I AM STILL UNSURE WHY LAS SENT ACR CROSSING UNDER US WHEN IT WOULD MAKE MORE SENSE FOR THEM TO HAVE USED RWY 19R AND US RWY 19L. NO CROSSING CONFLICTS.