The northern Vietnamese winter was especially cold this year, and only a few days before I crossed the pass into Sapa there had been black ice on the road. I was lucky enough to enjoy a few warm, sunny days in Sapa in February after a cold, misty ride up from Hanoi.
While the temperatures are never that low, even high in the mountains, the humidity is still high so it feels hell cold. I wear full textile riding pants and jackets over the winter here, with one or two scarves and ski gloves, and sometimes have to wear thermals and a windstopper.
There is usually a lot of fog at high elevations during the winter, so visibility is poor and the mist cuts right through to your skin.
My advice is to ride in southern Laos or Cambodia in February, when those countries are in the dry season, and save northern Vietnam for April to June and September through to about mid-December.