Why do some picklists have the blank value but others do not in Vault?
By design, all single-value picklists get a blank value so that users select it. Multi-value picklists do not use a blank value because they can be removed by clicking the X on all of the selected values. In a single value picklist, the user is able to choose a none value. Vault provides the blank space option to set it back to none/null/un-set.