I would have to say it's probably the fault of the website owner (vonage-promotion.com isn't run by Vonage, but by an affiliate trying to rack up refereral income) for not making it clear that the offer only applies to Canadian users.
Of course this doesn't help you now, but generally if you find a current Vonage user (like myself), he or she can refer you, and you would then get the second month free if use the referral invitation e-mail they send to you. Likewise, the existing user gets a credit towards a future bill for referring you.
I haven't actually referred anyone yet (though if anyone's interested in giving it a shot, by all means let me know :) ), but those who have say that they do get the credit as promised, and the referred customer likewise gets their free second month as promised.