B737 IN DSCNT FOR SNA IS GIVEN CONFUSING ATC INSTRUCTIONS DURING VECTORS.

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ON DSCNT TO SNA; WE WERE ON THE ARR TO THE ARPT. AT AN ALT OF APPROX FL200; WE WERE GIVEN A HEADING OF 130 DEGS TO FLY. A FEW MINS LATER; APCH OR CTR SAID 'CROSS MERMA AT 14000 FT.' APPROX 15 MI TO MERMA; STILL ON THE 130 DEG HDG; I INQUIRED THE CAPT IF ATC STILL WANTED US ON THE 130 DEG HDG OR DIRECT MERMA. THE CAPT ASKED ATC. THE RESPONSE WAS THAT WE WERE TO 'CROSS MERMA AT 14000 FT AND SHOULD HAVE BEEN DIRECT MERMA.' OUR 130 DEG HDG WOULD PUT US 5 MI AWAY FROM MERMA. I IMMEDIATELY INITIATED A TURN TO MERMA. WE WERE LEVEL AT 14000 FT AT THIS POINT. THE CTLR SAID TO FLY HEADING 160 DEGS. APPROX 15 SECONDS LATER; THE CTLR SAID TO FLY HEADING 170 DEGS. WE COMPLIED WITH BOTH VECTORS. ATC HANDED US OFF TO NEXT CTLR. WHAT WE DID WRONG: 1) WE SHOULD HAVE CLARIFIED THE 'CROSS MERMA AT 14000 FT' BY ASKING IF THEY WANTED US 'DIRECT MERMA' ALSO WHEN WE INITIALLY RECEIVED CLRNC. 2) WE SHOULD HAVE QUERIED ATC EARLIER IN THE DSCNT REGARDING THE 130 DEG HDG. WHAT WE DID RIGHT: 1) WE DID NOTICE THE DISCREPANCY; ALBEIT NOT EARLY ENOUGH; AND QUERIED THE CTLR. 2) REMAINED PROFESSIONAL AT ALL TIMES. NOT THE TIME TO CHANGE BARBS OVER THE RADIO. WHAT I LEARNED: DOUBLE AND TRIPLE CHK ALL CLRNCS. IF THEY DON'T ADD UP; ASK!