your problem boils down to doing things before the init
hook was fired. You should never check for code existence, DB structure before it. In case of frontend related code you should not do anything before template_redirect
hook.
Your code should be like
add_action('wp_head', 'myoverride', 1);
function myoverride() {
if ( class_exists( 'Vc_Manager' ) ) {
remove_action('wp_head', array(visual_composer(), 'addMetaData'));
}
}