Hi Brynn,
From my experiences with google group lists, I'd guess Marcel is a list subscriber, and his vacation auto-responder isn't clever enough to either recognise a list post and not respond at all, or to use the "reply-to" address which is the list's address; instead it's using the "from" address, which is you.
So I think there's nothing you can do, and no reason to worry about it. Hopefully when Marcel gets back from holiday he can fix it up..
FYI just using "reply" will send your message to the conversation on the list itself, which is probably what you want; reply-all will also email the author of the last message directly.