We could have a mail field indicating if vacation replies are wanted.
X-WantVacationReplies: no X-WantVacationReplies: noThese exist!
https://stackoverflow.com/a/10839343/1250772
This answer points at RFC containing recommendations for auto-reply e-mails, and specifies a header to use.
Any new proposal will fall victim to the same opt-in problem to an even greater degree; if developers of auto-responders can't be bothered to follow this RFC, they will also ignore even newer recommendations.