A B717-200 returned to the departure airport due to fuel imbalance and flight control vibration issues. Discovered right overwing fuel cap was missing.

Narrative

Aircraft was properly configured for taxi and takeoff. During climbout following gear/flap retraction I felt vibration in the control yoke. After checking aircraft cleaned up; we attempted to troubleshoot the problem. EAD alert revealed a fuel imbalance of approximately 1600-1700 pounds between right/left fuel tanks. Fuel levels between tanks appeared to continuously vary from 300 to 700 pounds differential through remainder of flight. Captain began crossfeeding operations to balance tanks as I continued climbout. Flight control vibration continued and appeared to increase in intensity. I recommended we return to field. Captain contacted ATC for return and we were vectored onto downwind for landing. No emergency declared nor deemed warranted and burned down fuel to below maximum landing weight. Accomplished all normal checklists and performed normal; uneventful approach and landing. Taxied to assigned gate. At the gate; a passenger reported fuel was streaming from the top of the right wing. Maintenance said the overwing fuel cap was missing.