An HTC Vive base station has a 3.5 mm jack that can be used as synchronization output. Base stations are usually used in pairs, with one set as mode-B and the other as mode-C. You can connect the base station set to mode-B to the VESA Stereo In connector on the Vicon Lock unit, to provide a genlock source.
(The following image shows the rear panel of a Lock+.)
Using the Steam VR signal to genlock the Vicon Vantage system offers the benefit of eliminating optical interference between illuminating sources, for example, Vicon camera strobes and the HTC Vive base stations.
In your Vicon application software, you can phase-shift the time when the camera strobes turn on for each frame with respect to the incoming Steam VR signal, by adjusting the VESA Offset property. This enables the strobe to be turned on at a different time from that of other potentially interfering sources.