You can’t use events to do what you need because rules are executed and the responses created at the post office level.
There may be a couple of possibilities. If the standard GroupWise classifications (not custom ones) are suitable for your needs, then you can set the default GroupWise classification (using the Client or an applet) for the account which has the vacation rule enabled. Every rule generated reply would then include the selected GroupWise classification within its body text.
The other option is to create a custom ‘out of the office rule’ applet which inserts classification text within the reply action.