wikipediavector.user.js

// ==UserScript==
// @name         Wikipedia Vector
// @author       Vusys
// @match        *://*.wikipedia.org/*
// @exclude      *://*.wikipedia.org/
// @grant        none
// @license      MIT
// ==/UserScript==

(function() {
    const url = new URL(window.location.href);

    if(url.toString().indexOf('useskin') === -1){
      	url.searchParams.set('useskin', 'vector');
        location.replace(url);
    }

    const a = document.getElementsByTagName('a');

    for (var i = 0; i < a.length; ++i){
        if(a[i].href.indexOf('wikipedia.org') === -1){
            continue;
        }

      	let href = new URL(a[i].href);
        href.searchParams.set('useskin', 'vector');      
        a[i].href = href;
    }
})();

Thanks for this lol. Hate the redesign.