You don’t explicitly set a slug, so it uses the post type key you registered it with. To set it to gallery, set slug
of rewrite
parameter:
$args = array(
'rewrite' => array( 'slug' => 'gallery' ),
// the rest of your args...
);
register_post_type( 'vmgallery', $args );
See the Codex page for register_post_type
for a full list of parameters.