Question:
Why the Parent Object Type fields in Child Object cannot be modified in Vault?

Answer:
By design, Vault does not support the reparenting of any Child Object records.
As a result, the Parent Object Type Field cannot be edited in the Child Object record by Vault Owners.
If the user wants to update the value of the Parent Object Type Field in the Child Object record, the record needs to be deleted and recreated with a correct parent record.
Related Documentation:
N/A