The point is that the two appear identical from many external signs. Sophisticated research suggests tests that "may" discern a difference, but the proof of the pudding is always in retrospect -- the patient "comes back" and tells what happened.
Actually the 2 are very different and are easily distinguished. In the case of PVS, there is no cortical function. That is a permanent condition and is clearly shown in tomography, especially MRI. Not only is there an abscence of any awareness, or consciousness in PVS, the patient is blind and has no cortical control of muscle. PVS results when everything above at least the thalamus is effectively dead.
Locked in syndrome results when cortical function is present and can easily be demonstrated with MRI, but connection is more, or less cut off to command muscular action. Regardless of the claims of some docs, this condition is common in patients during surgery. EEGs are present and the various features can be observed and distingushed in locked in. Their is no EEG in PVS.