I had a first date with a guy once. Went like this:
Him: fancy coming to mine for a coffee?
Me: It's late, no thanks.
Him: It's ok....I've had a vasectomy!

Got that one out the way...and no second date! And no I didnt turn down a second date because I was hoping for babies with him....

Seriously now, personally I would say that the best time is any time the subject, however vague, comes up. Probably not long into the relationship, you'll be at a diner one day and some horrid little kid will be acting up on the next table, and you can say, "Makes me glad I've had a vasectomy....another spoon of sugar darling?"

I know some people who put "dont want kids" on their dating website profiles.
That way you dont have to discuss it, unless they ask.
In the past when I;ve been approached by a fairly young/never been married or had kids guy showed an interest in me, one of the first things I said to them was "You sure you want to get involved with me? I wont be having any more kids".

Not sure if any of my rambles has helped...