List posts in metabox

I want to have a metabox in de post admin area with a dropdown list of some custom post type. To make a dropdown list I need to retrieve all custom post type outside the loop in my functions file. I use this code to get the post list

<ul>
<?php
global $post;
$args = array( 'post_type' => 'vacature');
$myposts = get_posts( $args );
foreach( $myposts as $post ) :	setup_postdata($post); ?>
	<li><?php the_title(); ?></li>
<?php endforeach; ?>
</ul>

I want the titles of the posts to store in a variable so I can use that variable as option for the select in my metabox. Is it possible to store this line of code
<li><?php the_title(); ?></li> in a variable as an array so I retrieve all custom post types?