Recently I was trying to use Oracle BPM 12c vacation rule but it was not working for me. I also posted on Oracle community and following was the detailed problem.
Oracle BPM 12c vacation rule issue
How it worked for me then? So go to your .task file of that user like manager in my case as shown below in snapshot.
Now double click on default.DefaultPerformet activity and go to properties. Then from General tab select names and expression from Build a list of participants from using dropdown list. Now at the bottom under participant name create a new participant i.e manager in my case as shown below.
Now save all the changes and redeploy the application. This time vacation rule will work fine.