Do not lose this letter. Reports indicate that this only drops the first time you defeat VanCleef. People who have killed VanCleef again, whether or not they had dropped the quest, have reported not being able to get the letter a second time. This quest starts the Lescovar Incident quest chain. The Defias Brotherhood quest chain leads you to defeating Edwin VanCleef in the Deadmines. It is not necessary for you to have taken on the quest to kill VanCleef for you to get the letter.