It's okay. [very happy] and [belated] both modify birthday.
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CliveThat's exactly the direction I was going to go with this. "Belated greetings for a happy birthday" is much better than t
However, I think it's not the birthday that is belated, but the birthday wish/greeting. To show this, you might say
I wish you a belated "Happy Birthday".
Best wishes, Clive