Preumably the 'roll numbers' (whatever they are) are the keys into the VSAM file. As the VSAM file is in key sequence you should first sort your flat file into the key sequence. Then read the flat file a record at a time, use the key field from there to retrieve the record with that key from the VSAM file. If the record is found - do whatever, if not found then do something else. Does that sound logical to you?
The problem I have is that people can explain things quickly but I can only comprehend slowly.