Air carrier First Officer reported smoke in the aft lavatory and cabin due to a passenger smoking in the lavatory. Flight returned to the departure airport.

Narrative

Climbing through FL300; we received a chime from the cabin. The Captain picked up the call; and I selected the cabin so I could listen. The purser was the one who called; from the forward cabin stating we have an indication of fire in the aft lav; and smoke coming out from under the door". At this time the Captain and I looked at each other. Ran through a few scenarios with the information provided and decided a return to field and requesting priority handling was appropriate.I requested priority handling; and informed ATC we would be making an immediate return to the airport. ATC cleared us direct the airport and maintain 12 thousand. ATC then requested fuel on board in pounds and sob. I responded 36400 lbs and XXX souls on board. We coordinated duties in the decent; communicated with the Dispatcher; loaded the FMS for direct to the final approach fix on Runway XX; performed a decent checklist; and vectored around a little weather on the way back to the airport. Descending through 14000; the purser called to inform us that there was an individual smoking/vaping in the aft lav and this is why we had the indications. We landed and taxied to the gate. We spoke to the chief pilot and were removed from the trip. At the time we were removed from the trip; I was 50/50 on if I should continue. I am grateful for the questions I received from the chief pilot and the process of being removed. After getting home 3 hrs later I was. 100% fatigued and on that 5.5 hr leg would have been 100% fatigued before arrival at the destination. The Captain and I did a great debrief; and discussed how the scenario played out."