To get data for a single vessel, first you need to find out its Vessel ID.
The Vessel ID is an internal number used by the FleetMon database to identify vessels. Every vessel that receives an entry in the database gets a Vessel ID assigned to it.
In order for you to get the proper response, you need to use the Vessel ID (not MMSI) within the search and then perform the query with the Vessel ID.
Use the Vessel Search API to get the Vessel ID for your desired ship.
To see how you can make an API call, have a look at this example.