Question:

When filtering a task view, the list of available filters will vary based on the kind of tasks that are assigned or available to the user. If no tasks are assigned to the user, the user should see all filters. This occurs because the user has no assigned or available tasks that Vault can use to logically limit the available filter list. But why the user cannot see "Created By" in the filter even though the user has no tasks?

Answer:

The user doesn't have permission for User Filters, hence filter that exposes user names will not be available.

Admin can navigate to Admin-->Users & Groups-->Permission Sets-->Application and select User Filters.

Related Documentation:

Vault Help: About Permission Sets