While you are on vacation, the opponent's clock ticks down normally when it is his move. So the opponent will not get any more thinking time for his next move just because you are on vacation.
Would you agree that this information removes the need for your proposed change?
I think that when someone takes Vacation time, it should be treated like adjournments used to be. The player with the move would record his move and seal it, unseen by the other player, with the benefit that both players would be unable to continue analysing the position during the downtime (beyond a certain point anyway--as both players are unable to say what the other is about to do).
This could be accomplished here by adding in the ability to input your move after clicking on Vacation, but have it stay "sealed" until you return from Vacation. As it is currently, we have to make a move in order to have our Vacation time activated, thereby giving our opponent the opportunity to study the position and plan his next move with the benefit of our Vacation time.
Thanks!