How to handle vacation is an open discussion, and not an easy one to solve. The thing is, vacation feature is there to give people an instant way out when they have an emergency or exceptional routine change IRL.
This is my humble opinion in this regard: Banning vacation is not the way to go. Correspondence players should embrace the posibility of their opponents going on vacation, the same way as they could come to a certain point themselves where using the vacation feature is the only way not to lose some games.
So, we should separate the vacation feature from the problem of finishing a correspondence game after a certain amount of time. Again, this is my opinion, and this is still an open discussion.