In a salutation on a line by itself, which version is punctuated correctly?
Hi, Mr Lawler,
I am writing to say [...]
Hi Mr Lawler,
I am writing to say [...]
Hi, Mr Lawler.
I am writing to say [...]
In a salutation on a line by itself, which version is punctuated correctly?
Hi, Mr Lawler,
I am writing to say [...]
Hi Mr Lawler,
I am writing to say [...]
Hi, Mr Lawler.
I am writing to say [...]
None of them are punctuated correctly, according to American English conventions, because abbreviations must be followed by a full stop. In British English, no full stop following Mr is perfectly fine, however.
According to the Purdue OWL, business letter, i.e. very formal, salutations must be followed by a colon (:). And the comma between the title and interjection depends on whether the noun is being directly addressed (or as you said: in the vocative case). So, Dear Mr. Lawler: requires no comma because it is an adjective modifying the noun, whereas Hello, Mr. Lawler: requires a comma because it is directly addressing the noun.
So, this would be the suggested usage, based on those guidelines:
Hi, Mr. Lawler:
I am writing to say [...]
The Purdue OWL also mentions a less common format, known as open punctuation, whereby no punctuation is present after the salutation and valediction:
Hi Mr Lawler
I am writing to say [...]
Thank you
[Your name]
Please note that "Hi" is can be interpreted as somewhat of an informal greeting and should probably be replaced by a more formal greeting like "Hello" or "Dear".