The Tampermonkey userscript to download video and audio from Youtube, Twitter, Vimeo, Facebook, Dailymotion, 1tv, VK, youku, bilibili and 5000 more sites for free. Fast and easy to use.

Video_Audio_Downloader.js

// ==UserScript==
// @name         Distill Video & Audio Downloader from 5000+ sites including Youtube, Support 1080P, 2K, 4k & 8K
// @namespace   https://distillvideo.com/
// @version     2.1.1
// @date        2018-06-17
// @description Browser extension to download video and audio from Youtube, Twitter, Vimeo, Facebook, Dailymotion, 1tv, VK, youku, bilibili and 5000 more sites for free. Fast and easy to use.
// @author      DistillVideo.com
// @copyright   2018, DistillVideo.com
// @homepage    https://distillvideo.com/page/extensions
// @downloadURL https://distillvideo.com/js/ditillvideo.user.js
// @compatible chrome
// @compatible firefox
// @compatible opera
// @compatible safari
// @license GNU GPL v3.0 or later. http://www.gnu.org/copyleft/gpl.html
// @match          *://*.youtube.com/*
// ==/UserScript==

(function() {
    'use strict';

    if (document.getElementById("polymer-app") || document.getElementById("masthead") || window.Polymer) {
    setInterval(function() {
        if (window.location.href.indexOf("watch?v=") < 0) {
            return false;
        }
        if (document.getElementById("count") && document.getElementById("distillvideo") === null) {
            Addytpolymer();
        }
    }, 100);
} else {
    setInterval(function() {
        if (window.location.href.indexOf("watch?v=") < 0) {
            return false;
        }
        if (document.getElementById("watch7-subscription-container") && document.getElementById("distillvideo") === null) {
            AddhtmlDV();
        }
    }, 100);
}

function AddhtmlDV() {
    if (document.getElementById("watch7-subscription-container")) {
        var wrap = document.getElementById('watch7-subscription-container');
        var button = "<div id='distillvideo' style='display: inline-block; margin-left: 10px; vertical-align: middle;'>";
        button += "<a href=\"https://distillvideo.com/?url=https://www.youtube.com/watch?v=" + disvidyouvid(document.URL) + "\" title=\"Download this video\" target=\"_blank\"" +
            "style=\"display: inline-block; font-size: inherit; height: 22px; border: 1px solid rgb(0, 183, 90); border-radius: 3px; padding-left: 28px; cursor: pointer; vertical-align: middle; position: relative; line-height: 22px; text-decoration: none; z-index: 1; color: rgb(255, 255, 255);\">";
        button += "<i style=\"position: absolute; display: inline-block; left: 6px; top: 3px; background-image: url(data:image/svg+xml;base64,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); background-size: 12px; background-repeat: no-repeat; background-position: center center; width: 16px; height: 16px;\"></i>";
        button += "<span style=\"padding-right: 12px;\">Download</span></a></div>";
        var style = "<style>#distillvideo button::-moz-focus-inner{padding:0;margin:0}#distillvideo a{background-color:#15388c}#distillvideo a:hover{background-color:#E91E63}#distillvideo a:active{background-color:rgb(0, 151, 74)}</style>";
        var tmp = wrap.innerHTML;
        wrap.innerHTML = tmp + button + style;
    }
}

function Addytpolymer() {
    var buttonDiv = document.createElement("span");
    buttonDiv.style.width = "100%";
    buttonDiv.id = "distillvideo";
    var addButton = document.createElement("a");
    addButton.appendChild(document.createTextNode("Download video"));
    addButton.style.width = "100%";
    addButton.style.backgroundColor = "#15388c";
    addButton.style.color = "white";
    addButton.style.textAlign = "center";
    addButton.style.padding = "5px 10px";
    addButton.style.margin = "0px 10px";
    addButton.style.fontSize = "14px";
    addButton.style.border = "0";
    addButton.style.cursor = "pointer";
    addButton.style.borderRadius = "2px";
    addButton.style.fontFamily = "Roboto, Arial, sans-serif";
    addButton.style.textDecoration = "none";
    addButton.href = "https://distillvideo.com/?url=https://www.youtube.com/watch?v=" + disvidyouvid(window.location.href);
    addButton.target = "_blank";
    buttonDiv.appendChild(addButton);
    var targetElement = document.querySelectorAll("[id='count']");
    for (var i = 0; i < targetElement.length; i++) {
        if (targetElement[i].className.indexOf("ytd-video-primary-info-renderer") > -1) {
            targetElement[i].appendChild(buttonDiv);
        }
    }
}

function disvidyouvid(url) {
    var p = /((http|https)\:\/\/)?(?:[0-9A-Z-]+\.)?(?:youtu\.be\/|youtube(?:-nocookie)?\.com\S*[^\w\s-])([\w-]{11})(?=[^\w-]|$)(?![?=&+%\w.-]*(?:['"][^<>]*>|<\/a>))[?=&+%\w.-]*/ig;
    return (url.match(p)) ? RegExp.$3 : false;
}

})();

But I don't recommand this as it is outdated and the service is using is down... so you better find some other script, tell me what website are you looking to download from and I will tell which one is better, for Youtube the best option is Local Youtube Downloader

@neoOpus trying for VK.com . Used to be OK with some hoop jumping but finding it very challenging now. Thanks

Button didn't appear, Site didn't work.

Doesn't work for any site just tried it.