When you call FMSOV or FMSOV2 to create a vector file, FMS completes the Vector File Attribute List LUX(25), which you provide. This array contains all the information FMS needs to manage the vector file.
Each vector is stored as a record on file LUX(1). The record length is computed by FMS and stored in LUX(4). You should always use this record length when addressing the file to store multiple vectors.