Hi Francis,
the autoreply message you receive is sent from the real user, not in the name of the group, so the problem you foresee should not occour (if I understood your point). Moreover, the autoreplies are always sent one each X days, and the purpose of this is (also) to avoid that two email systems end up in an endless autoreply loop...
I found a way to have the vacation message sent as I wish with the help of Dungog guys when I posted this question in bugzilla, you can have a look at bug # 3148 (even though it's not a real bug).
Bye!
Mauro