VSAM files are used when we want to store data based on some key fields(Eg: KSDS)access method is indexed.NON VSAM files like PS are used when we want to store data which is not unique and also the access is sequential.
In VSAM files records are stored in control intervals where as in FLAT files records are stored in blocks.
FLAT files are sequential read . VSAM files can sequential and random read.
In VSAM access is fast because of organization of records.