Reassign a task
There is no-real-way of doing this by default. You will have to create business process to do it as every business and the requirements for an Auto-reassign is different.
How do you know when the manager is on holiday to reassign? You'll need to interrogate the HR system...etc...
PS: While I understand this may not be the answer you are looking for it does answer your question, please mark as resolved by ticking the tick mark on the left of this reply so others now it is closed.