If you’re on Unity 2018, you can grab the latest CM release candidate from the package manager. In it, there is a new API call: vcam.OnTargetObjectWarped. After repositioning your target, you would call it as follows (transform is the thing that got moved):
// Inform any vcams that might be tracking this target
int numVcams = CinemachineCore.Instance.VirtualCameraCount;
for (int i = 0; i < numVcams; ++i)
CinemachineCore.Instance.GetVirtualCamera(i).OnTargetObjectWarped(
transform, posDelta);