Force autoplay=0 (off) for a Vimeo video that opens in an iframe tag.

force-vimeo-autoplay-0.js

(function () {
  document.addEventListener("DOMContentLoaded", function () { // Wait for all the contents to load.
    const elts = document.querySelectorAll("figure a"); // Select all video gallery links.
    if (!elts.length) return; // If nothing, bail early.
    [...elts].map((elt) => { // Loop over each link.
      elt.addEventListener("click", function (e) { // Listen for a click on each gallery link.
        console.log("Got a click on a gallery item.");
        setTimeout(function () { // Wait for the iframe b/c it loads dynamically.
          console.log("Overwriting the autoplay to 0!"); 
          let iElt = document.querySelector("iframe");
          console.log(`iframe src before ${iElt.src}.`);
          iElt.src = iElt.src.replace("autoplay=1", "autoplay=0"); // Force the autoplay to 0 by overwriting the query param.
          console.log(`iframe src after ${iElt.src}.`);
        }, 2000); // Tweak this wait time as needed (set to 2 seconds right now).
      }); // listener
    }); // map
  });
})(); // iife

This example code turns off autoplay for Vimeo videos that open in an iframe tag. It's useful if you don't have access to the Vimeo embed code.

You'll need to tweak the timeout as needed.