Found and answer here: https://github.com/CompassHB/web/issues/67#issuecomment-245857301

The example below is taken from the link above:

/** * Modify REST API content for pages to force * shortcodes to render since Visual Composer does not * do this */add_action( 'rest_api_init', function (){   register_rest_field(          'page',          'content',          array(                 'get_callback'    => 'compasshb_do_shortcodes',                 'update_callback' => null,                 'schema'          => null,          )       );});function compasshb_do_shortcodes( $object, $field_name, $request ){   WPBMap::addAllMappedShortcodes(); // This does all the work   global $post;   $post = get_post ($object['id']);   $output['rendered'] = apply_filters( 'the_content', $post->post_content );   // EDIT: add custom CSS to $output:   $output[ 'yb_wpb_post_custom_css' ] = get_post_meta( $object[ 'id' ], '_wpb_post_custom_css', true);   return $output;}

EDIT

A question arose in a comment: how to get the custom CSS set for a page (post, etc.)? I modified the sample code in a way that it adds the custom CSS to the REST API response. You'll find the CSS in content/yb_wpb_post_custom_css.

The other way is to add another field to the REST API response that contains this CSS. The key is that the custom CSS set for a page/post/etc. has a meta key _wpb_post_custom_css.

About 2 years late to the party, however, the following worked for me:

$output['rendered'] = apply_filters( 'the_content', get_the_content() );

Just in case if anyone was wondering.