Skyhawk 172/Cutlass 172
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 10
Communication Breakdown; Time Pressure
ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Air Traffic Control Provided Assistance
Human Factors; Environment - Non Weather Related; Aircraft; Airspace Structure
Aircraft X was released for departure off of SDM on the sole DVA of heading 280.  After I released him a VFR target that had been maneuvering offshore at 4;700 ft began to descend and fly towards SDM.  Aircraft X departed just as the VFR was descending through 2;700 ft. I called SDM tower to see if they were talking to that traffic to see if they could stop the descent but they were not talking to him. Aircraft X checked on with me; I stopped his climb at 2;000 and called traffic; hoping the target would stop descending or that Aircraft X would see him.  Neither occurred; so I turned Aircraft X left to avoid traffic; below the MVA. Aircraft X was at 1;700 ft in a 1;900 ft MVA.  I should have called SDM tower sooner to see if they could just hold the departure until we saw exactly what the VFR traffic would do.
SoCal TRACON Controller reported vectoring an aircraft below the MVA to avoid traffic.