You can assign a default user for approval associated to a supplier. In
the partner form view, go to the *Sales and Purchase* tab, and into the
*Purchase* section, and fill the field *Approver of Vendor Bills*.

When you create a vendor bill the field *Responsible for Approval* will
be filled in with the partner's default. You can change it if needed.

Be aware that you won't be able to post a vendor bill unless you have
indicated a Responsible for Approval.

Used in connection with the module *Account Move Tier Validation* you
can set up approvals specific to a department.