I remember an occasion where I felt pleased by such a "forum birthday greeting", even though I knew it was automated. Not very rational I guess.
Anyway, one user complained about his birthday email now and on second thought *, because I want the forum and everything else to be authentic, and because it might actually annoy some users, I disabled these birthday greetings now.
That means no automated birthday greetings anymore from now on.
* Actually, first thought, as it is enabled on all vBulletin forums by default.