i want that in the fixed file i have in the first position the logical length of the varible file (vb).
I guess you meant you want the readable logical length starting
in the first position. The RDW in the first four bytes actually contains the binary length in the first two bytes and zeros in the third and fourth bytes. You can use a DFSORT job like the following to do what I think you're asking for:
//S1 EXEC PGM=SORT
//SYSOUT DD SYSOUT=*
//SORTIN DD DSN=... input file (VB/618)
//SORTOUT DD DSN=... output file (FB/618)
//SYSIN DD *