I come not praise the vocative, but to bury it. That once mighty case of address, cut down in its prime by the informality of email.
In elementary school, I was taught to begin the composition of letter with the phrase "Dear Person,". That's 'dear' + the recipient's name + comma. Dear is an adjective that modifies the noun. They cleave together and form a single unit. The comma which follows indicates that I am addressing the recipient. Voila. The vocative case. Hey, you. Hi, Dad. Hello, dear friend. Good riddance, blue serpent.
Alas, "dear" sounds archaic at the top of an email. Now we are left with the hip, eworthy greeting "hi". Simple? One swaps the word "hi" for "dear" right?
Wrong, fool. "Hi" is itself a greeting that requires the vocative. In other words, it must be followed by a comma. It does not form an adjectival phrase to be followed by a comma. That was the function of the now semi-defunct "Dear Person,".
It may seem harsh, but I use this as a quick 'test' of the sender whenever I receive an email message. Failing the test doesn't indicate that the sender is stupid (it is a common mistake) or that the content is not worthwhile. It does, however, indicate that linguistic prowess will not be found.