How to autoload vendor classes in a plugin?

<?php

namespace Grav\Plugin;

use Composer\Autoload\ClassLoader;
use Grav\Common\Plugin;
use Mollie\Api\MollieApiClient;

class MolliePlugin extends Plugin
{
    public static function getSubscribedEvents()
    {
        return [
            ['autoload', 100000],
            'onPluginsInitialized' => ['onPluginsInitialized', 0]
        ];
    }

    public function autoload(): ClassLoader
    {
        return require __DIR__ . '/vendor/autoload.php';
    }

    public function onPluginsInitialized()
    {
        if ($this->isAdmin()) {
            return;
        }

        $this->enable([
            'onFormProcessed' => ['onFormProcessed', 0]
        ]);
    }

    public function onFormProcessed($event)
    {
        $form = $event['form'];
        $action = $event['action'];

        switch ($action) {
            case 'mollie':
                $mollie = new MollieApiClient();
                // And so on
        }
    }
}

use Mollie\Api\MollieApiClient;
$mollie = new MollieApiClient();
// Class 'Mollie\Api\MollieApiClient' not found

use \Mollie\Api\MollieApiClient;
$mollie = new MollieApiClient();
// Class 'Mollie\Api\MollieApiClient' not found

$mollie = new MollieApiClient();
// Class 'Grav\Plugin\MollieApiClient' not found

$mollie = new \MollieApiClient();
// Class 'MollieApiClient' not found

$mollie = new \Mollie\Api\MollieApiClient();
// Class 'Mollie\Api\MollieApiClient' not found

$mollie = new Mollie\Api\MollieApiClient();
// Class 'Grav\Plugin\Mollie\Api\MollieApiClient' not found