B737-400 CREW HAD TRACK DEV IN ZLA AND ZLC CLASS A.

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ENRTE PHX TO PDX IN CRUISE AT FL350; CTR GAVE US SEVERAL VECTORS FOR TFC. AFTER THE VECTORS WERE NO LONGER NEEDED; WE WERE GIVEN 'DIRECT IMB' -- A VOR HUNDREDS OF MI AWAY. WITH OUR DUAL GPS NAV AND FMS SYS; AND ANTICIPATING THE 'DIRECT IMB' CLRNC; I QUICKLY SELECTED THE IMB WAYPOINT AND PUSHED 'EXECUTE;' THUS ESTABLISHING THE LNAV PATH DIRECT TO IMB. BUT; NEITHER I NOR THE FO SELECTED THE 'LNAV' MODE WHICH WOULD MAKE THE AIRPLANE FLY THE INTENDED PATH. ESSENTIALLY; WE STAYED IN 'HDG SELECT' BUT; AS THE HDG ASSIGNED WAS APPROX THE SAME AS THAT HDG 'DIRECT TO IMB;' NEITHER OF US NOTICED THE ERROR. EVENTUALLY; DUE TO COMPLACENCY; WE DID NOT NOTICE WE WERE BEING BLOWN OFF COURSE BY A XWIND AND ENDED UP EMBARRASSINGLY E OF THE ORIGINAL DIRECT PATH TO IMB. CTR CALLED US TO INQUIRE WHETHER WE WERE GOING 'DIRECT' AND WE NOTED OUR ERROR AND ADJUSTED. THE PROB WAS PROBABLY CAUSED PARTIALLY BY MY ENTERING THE 'DIRECT' ROUTING IN THE FMC; WHILE IT WAS THE FO'S LEG; SO WE EACH PROBABLY ASSUMED THE OTHER PLT WOULD SELECT LNAV. IT WAS A SUNNY; PLEASANT AFTERNOON; A SMOOTH RIDE; AND A STUPID; COMPLACENT MISTAKE!