Your "requirement" is so poorly defined that nobody could possibly help you much. When you say "VSAM file", do you mean an ESDS or KSDS or RRDS or LDS VSAM file? If KSDS, is the name field the key? If not, you must read the entire file sequentially and find each record having the required letter in the required column. What does a "flat file" (I assume you mean a sequential file) have to do with the VSAM file -- the two are not related? Sequential files have to be read one record at a time, anyway.
Furthermore, be aware that asking for logic and program code on this forum is asking professionals for their work product. This typically requires that you pay them for their time and expertise -- and rates are typically 1000 U.S. dollars (or euros) per day, although there are occasional offers as low as 500 per day.