ContactController.php

<?php

    //…
    /**
     * @Route("/contact", name="contact_new")
     * @Template
     */
    public function indexAction(Request $request)
    {
        $contact = new Contact();
        $form = $this->createForm('contact', $contact);
        $form->add('send', 'submit');

        $form->handleRequest($request);

        if ($form->isValid()) {
            $this->get('vendor_mail')->send($contact);

            return $this->redirect($this->generateUrl("contact_success"));
        }

        return [
            "form" => $form->createView()
        ];
    }

ContactType.php

<?php
    //…

    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('sender', 'text')
            ->add('subject', 'text')
            ->add('message', 'textarea')
        ;
    }

    public function getName()
    {
        return 'contact';
    }
}