A coauthor and I are drafting a letter, and we're not yet sure whom we're going to be sending it to. So I sent a draft to my coauthor, which started
Now, that line in a letter is in the vocative case (or would be, if English really had cases). I know that who is used in the nominative case and whom in the accusative/dative, but what about the vocative? Should I have written
Obviously, my question is only about such dialects as use who and whom. But it could be just as well asked about he versus him: had we been authoring a letter with a known male recipient we didn't want to bother writing out the name of, we could have used
and the same question would apply.
Equally obviously, because this is a draft letter and the word in question won't appear in the final copy, it really doesn't matter which word we use. I wish to know anyway.