Sort WP_Query with numeric custom field

by

Your meta key should be specified as the meta_key argument, and orderby should be meta_value_num:

$args = array( 
   'post_type' => 'post_sermons',
   'post_status' => 'publish',
   'posts_per_page' => ot_get_option('wpl_sermon_per_page'),
   'paged'=> $paged,
   'meta_key' => 'video_order',
   'orderby' => 'meta_value_num',
   'order' => 'ASC' 
);

See WP_Query for explanations of query arguments.