Actually I have a great idea ;)
Because Susan went to a vacation, let's enable the 'Alternate' greeting for her voice mail box.
When you enable alternate greeting it is overriding all other greetings enabled.
But when you enable it, set the 'Enabled Until' date & time. That'll be the date & time when she gets back.
Also, in the field named 'User with Mailbox', select John's voice mail box, and select 'Go directly to Greetings'
I think it should work great.
Now that I think about it, it almost the same like setting up a call forward from Susan to John, because at the end callers will hear John's greeting.
Here's an example: