In-vacuum Faraday isolators (FIs) are used in gravitational wave interferometers to prevent the disturbance caused by light reflected back to the input port from the interferometer itself. The efficiency of the optical isolation is becoming more critical with the increase of laser input power. An in-vacuum FI, used in a gravitational wave experiment (Virgo), has a 20 mm clear aperture and is illuminated by an almost 20 W incoming beam, having a diameter of about 5 mm. When going in vacuum at 10-6 mbar, a degradation of the isolation exceeding 10 dB was observed. A remotely controlled system using a motorized λ=2 waveplate inserted between the first polarizer and the Faraday rotator has proven its capability to restore the optical isolation to a value close to the one set up in air.