The system reports an uncorrelated vessel, but when the AIS layer is superimposed there is a reporting vessel next to it. Is this the same vessel?

Probably. The CMS service uses algorithms to decide if a vessel detected in an image is correlated with a vessel report. However, if the vessel report is too far from the detected vessel it might fail to correlate properly. This is more likely to happen with satellite AIS reports, as the frequency of reports is lower than for terrestrial AIS.