I have opened a ticket with Theme Forest to hopefully get the problem fixed.
The issue is caused by the WP Theater and Fusion Core shortcode for Vimeo and Youtube videos being the same.
You can fix the issue yourself by navigating to wp-content/plugins/fusion-core/shortcodes. Open up class-vimeo.php or class-youtube.php (or both if you use both sites), and delete the 3 lines in the ‘public function __construct()’ function near the top.
For example, you will delete these 3 lines in class-vimeo.php:
add_filter( 'fusion_attr_vimeo-shortcode', array( $this, 'attr' ) );
add_filter( 'fusion_attr_vimeo-shortcode-video-sc', array( $this, 'video_sc_attr' ) );
add_shortcode('vimeo', array( $this, 'render' ) );