A vibration measurement system and relative method to monitor multiphase flows in extraction wells or conduits, by analysis of backscattered multimode fiber light comprising: • a sensing multimode optical fiber; • an optical source, containing a high coherence laser that emits optical pulses to be sent in said sensing fiber; • a photonic lantern with 3 or more single-mode optical fiber ports and one multimode fiber port that is connected to the sensing multimode fiber; • an optical receiver, comprising a number of photodetectors equal to the number of the single-mode optical fiber ports of said photonic lantern minus 1, wherein each photodetector is connected to each of said single-mode ports; • a system for processing output signals from the optical receiver, the optical source being connected to one of the single-mode optical fiber ports and the other single-mode fibers being connected to the optical receiver. A process is also described for reconfiguring an optical reflectometry system already installed at a facility to be monitored.