Sierra libraries that are currently using the cumbersome ipsso integrated login page have to keep using that page if they move to Vega. Polaris libraries can use a unique to Vega login page/widget that needs little to no configuration. (Correct me if I'm wrong about that.) We would have loved to ditch our crusty old ipsso page and not have to worry about it anymore when we bring Vega online but as a Sierra library we don't have that option.