Bach Ma is the wettest place in Vietnam, with the heaviest of the rain falling in October and November (bringing out the leeches). It’s not out of the question to visit then, but check road conditions first. The best time to visit is from February to September, particularly between March and June.
At the visitor centre by the park entrance there's an exhibition on the park’s flora and fauna, and hiking booklets are available. You can book village and birdwatching tours and English-speaking guides (500,000d per day).