Response article to a flawed paper you reviewed

Wait. The authors have seen your review, they should know that they have a problem. There's nothing to be done until the flawed article appears in another journal (if it ever does). You might write a draft response article and sit on it until the unchanged article appears. If it does, you can update your draft based on any changes and submit it to the same journal in response. If they fix it before resubmitting it, then you have nothing to do, and your review will have been successful.