Learn English – Punctuating a salutation on a line by itself

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".