There aren't any true luxury resorts in the North as far as I know. The Mgallery Legacy Yen Tu (designed by Bill Bensley) and Melia Ba Vi Retreat look quite nice though. Both places are fairly new, not too far away from Hanoi and have pleasant weather in October. I've been thinking of visiting them the next time I'm in Hanoi. A bit further away is Mai Chau, also a mountainous area with a relaxing atmosphere. The nicest place to stay there is Mai Chau Ecolodge, the style is rustic and not luxury.