I completely agree with the use of a comma.
It is the effect of text messaging and social media I formal language that has made people follow a quick, ‘lazy’ form of writing.
Even language teachers are falling into this. Looking at languages text books, punctuation is used in such sentences. One has to look at dialogues, for example.
As mentioned above, meaning changes when there is no comma:
Hello Jane! sounds like saying Hello Hotel!
Excellent Ali! means Ali is excellent, not that he has done something well.
I would use the following:
Dear Leila,
Hello, Vlad!
Thank you, Marco!
Good evening, Yuko!
How are you, Miranda?
etc.