Horde vacation module does not work.
Non-supported third-party application.
Horde Vacation module is an addition to Horde webmail solution, not a part of it. This Vacation module is not supplied with Plesk Panel or with Horde by default in this matter.
You can find this information in vendor's official documentation to Vacation module.
So, the task of installing and configuring Horde Vacation module lies out of scope of Parallels Technical Support.