Change this line
add_theme_support( 'post-thumbnails');
To
add_theme_support( 'post-thumbnails', array('post', 'page','vacature'));
I think this will help you.
Change this line
add_theme_support( 'post-thumbnails');
To
add_theme_support( 'post-thumbnails', array('post', 'page','vacature'));
I think this will help you.