Options A, B, and D are not the first steps necessary for achieving this goal:
Option A: Creating a Vault in S3 Glacier is not directly managed from Veeam. Veeam interacts with Glacier through the Archive Tier in the SOBR.
Option B: Adding AWS S3 as a Capacity Tier is a prerequisite but not the first action needed if the Capacity Tier is already set. You need the Archive Tier to facilitate moving data to Glacier.
Option D: Reconfiguring the SOBR to use block storage as the Performance Tier is not relevant to the requirement of moving data to Glacier.
Therefore, the correct initial step is:
C. Add an Archive Tier with the appropriate Bucket from S3.