A B737-800's lavatory vacuum motor worked poorly on the ground. A Contract Mechanic applied the MEL incorrectly and during flight the lavatories filled. The flight crew discovered an incorrect circuit breaker was pulled preventing pressurization vacuum from operating the lavatories.

Narrative

Shortly before departure; we were notified by the flight attendants that all three lavatories were inoperative. After further investigation; we realized that the lavatories would flush but without much suction. Contract Maintenance was notified and the troubleshooting began. Ultimately; Maintenance Control and the local Mechanic employed the MEL placarding the lavatories on the ground; but they would be available in-flight. Mechanic pulled a 2.5 AMP circuit breaker labeled 'vacuum waste' behind the Captain and completed the logbook sign off. Thirty minutes into flight the flight attendants notified us the lavatories were worse than before and beginning to fill up. After closer inspection of the MEL; I realized the mechanic should have pulled a 20 AMP circuit breaker labeled 'vacuum toilet blower motor' behind panel P91 (later determined to be in the E&E compartment) not 'vacuum waste.' After receiving permission from Maintenance Control the 'vacuum waste' circuit breaker was reset and the lavatories began to function. Flight was continued to our destination.