Duc's Bistro   Pau hana, Duc's is a downtown power brokers hangout, and they linger for French and Vietnamese food blue crab cakes in Thermidor sauce or spicy lemongrass chicken? in the pre-fall-of-Saigon room, bathed in rose-pink light with art-lined walls and crisp white tablecloths. Owner Duc Nguyen is your distinguished host. The Angus steak au poivre is one of the best in town (come at lunch, and it's a flatiron for $20). But don't be fooled by the seedy street outside the prices here are all upscale.  1188 Maunakea St. 531-6325. 11:30 a.m.-2 p.m., 5-10 p.m. Mondays-Saturdays. Reservations recommended. AX, DS, JCB, MC, VS. No checks. $$$.