Check if a jQuery plugin is loaded. If not, load it using the $.getScript() function.

pluginLoaded.js

jQuery(document).ready(function ($) {

	'use strict';

	function initModal() {
		$('.popup-youtube, .popup-vimeo, .popup-gmaps').magnificPopup({
			disableOn: 700,
			type: 'iframe',
			mainClass: 'mfp-fade',
			removalDelay: 160,
			preloader: false,
			fixedContentPos: false
		});
	}

	// Check if magnificPopup is loaded
	if (jQuery().magnificPopup) {

		initModal();

	} else {

		// Load the stylesheet
		$('<link/>', {
			rel: 'stylesheet',
			href: pluginLoadedParams.styleUrl
		}).appendTo('head');

		// Load the script
		$.ajaxSetup({
			cache: true
		});
		$.getScript(pluginLoadedParams.scriptUrl, function () {
			initModal();
		});

	}
});

pluginLoaded.php

<?php
function pluginLoaded_scripts() {
	// Enqueue a script with jQuery as a dependency.
	wp_enqueue_script( 'pluginLoaded', get_template_directory_uri() . '/js/pluginLoaded.js', array( 'jquery' ), '1.0.0', true );

	// Define our parameters
	$params = array(
		'styleUrl' => get_template_directory_uri() . '/css/vendor/magnific-popup.css',
		'scriptUrl' => get_template_directory_uri() . '/js/vendor/jquery.magnific-popup.min.js',
	);

	// Create the JavaScript object
	wp_localize_script( 'pluginLoaded', 'pluginLoadedParams', $params );
}

add_action( 'wp_enqueue_scripts', 'pluginLoaded_scripts' );

Good stuff, thank you :)