The reflected light from the sail wouldn't be much help.
Especially if the vehicle is a long way off, because of light-travel time. If the vehicle started to drift off the beam, by the time the information got back to the laser operators, and the beam was re-directed, the vehicle would have drifted even further.
You can't get any information back from the vehicle faster than light, and then you have to multiply that by two to get the corrected beam back to the spacecraft.