You can contact him at the link below. Tell everyone you know to email him to demand that they allow the current vendors to stay. There is very little of the sublime that is still left in NYC. Let this one bit stay.
From everything I know about advocacy (not a terrible amount, but still...), the most effective protest is not email, but letters. Specifically, handwritten ones (if your handwriting isn't too bad). Emails often get written off as just slactivism. Anyway, regardless, take a chance to go do the soccer fields, and see for yourself that it's worth saving.