How do I define a holiday replacement for users. Ideally configurable by the use

An enhancement request is in place for a feature that will allow users to select their proxy when they are gone for vacation.

Your reference number for this request is: #22185

For your questions, I suggest building a process which your users will have access to where they can manage their replacements and their time off themselves (perhaps as quick tasks). This can be used in conjunction with the Task Services Plugin, available under Components --> Smart Services. You can build a process to programatically reassign tasks. When a user goes out of office, their tasks can bereassigned with this smart service.