A B727 Engineer reported that the Captain; on duty for fourteen hours; failed to reselect the correct NAVAID allowing the aircraft to deviate before ATC alerted.

Narrative

ATC had given us a vector for an enroute VOR. As we got into the service volume of the VOR; the Captain began to navigate directly to the VOR. While preparing for the STAR into our destination; the Captain mistakenly tuned the #1 VHF navigational radio to a NAVAID on the STAR and did not return it to station we were cleared to. He began errantly navigating to the wrong VOR. Neither the Copilot nor I (Engineer) noticed the changed frequency as we were involved in other flight deck duties. When ATC questioned us as to where we were navigating; it was obvious that we would pass nearly 30 miles south of the VOR. We made about an eight degree course correction to proceed to the assigned VOR. Captain and First Officer were on duty for approximately 14 hours and had been awake approximately 20 hours at the time of the incident.