A vehicle security system may include at least one security sensor and a security controller connected thereto, and a siren which also serves as a shock sensor. The siren may include a housing, a siren security electrical signal generator circuit carried by the housing for generating an electrical siren security alarm signal responsive to the security controller, and a shock detector circuit carried by the housing for processing an electrical shock sense signal for the security controller. The siren also preferably includes an electrical/mechanical (E/M) transducer carried by the housing for sounding a siren security alarm responsive to the electrical siren security alarm signal, and for generating the electrical shock sense signal responsive to mechanical shock. In other words, the E/M transducer and associated circuitry provide the dual alternate functions of sounding the siren security alarm and sensing for shocks to the vehicle.