There is nothing built in for this right now. You can, as Joram pointed out, query all tasks assigned to user X and call a "delegate" action on the task. When delegating, the original assignee (the user that is on vacation) is retained in the owner-field of the task. Later on, you can resolve all delegate tasks again.
If you want a generic solution, not depending on N+1 rest calls, you can create a custom rest-resource in the Acitivit-rest project and do this logic in a single rest-resource.