Link to a admin submenu item using a custom link

If I understand correctly what you are asking, you need the $menu_slug used when adding your menu pages:

//add_menu_page( $page_title, $menu_title, $capability, $menu_slug, $function, $icon_url, $position );
//add_submenu_page( $parent_slug, $page_title, $menu_title, $capability, $menu_slug, $function );

$your_link_url = admin_url( 'admin.php?page=".$menu_slug, "http' );

OR SPECIFICALLY FOR YOU

$your_link_url = admin_url( 'admin.php?page=nes_vendor_new', 'http' );

You can always verify the link by clicking on the menu item and see where it takes you.