The ESO Very Large Telescope Interferometer (VLTI) offers access to the four 8-m Unit Telescopes (UT) and the four 1.8-m Auxiliary Telescopes (AT) of the Paranal Observatory. After the first fringes obtained in 2011 with the commissioning instrument VINCI and with siderostats, the VLTI has seen an important number of systems upgrades, paving the path towards reaching the infrastructure level and scientific results it had been designed for. The current status of the VLTI operation all year round with up to four telescopes simultaneously and real imaging capability demonstrates the powerful interferometric infrastructure that has been delivered to the astronomical community. Reaching today’s level of robustness and operability of the VLTI has been a long journey, with a lot of lessons learned and gained experience. In 2007, the Paranal Observatory recognized the need for a global system approach for the VLTI, and a dedicated system engineering team was set to analyse the status of the interferometer, identify weak points and area where performances were not met, propose and apply solutions. The gains of this specific effort can be found today in the very good operability level with faster observations executions, in the decreased downtime, in the improved performances, and in the better reliability of the different systems. We will present an historical summary of the system engineering effort done at the VLTI, showing the strategy used, and the implemented upgrades and technical solutions. Improvements in terms of scientific data quality will be highlighted when possible. We will conclude on the legacy of the VLTI system engineering effort, for the VLTI and for future systems.