Madagascan NGO Fanamby manages the Loky Manambato Protected Area, 56km west of Vohémar, to support local villages and protect flora and fauna. Most spectacular are the critically endangered golden-crowned Tattersalli sifakas: about 1000 of them live only in the seven forested areas of this reserve. Hire a Fanamby guide through Hôtel Lémurien Blanc in the village of Daraina (the price should be no more than Ar15,000 plus entry permit Ar10,000) or stay at Fanamby's new Camp Tattersalli in the forest. Set out early, as the lemurs sleep during the heat of the day. It's a 50 minute drive from Daraina into the forest, then 20-30 minutes' easy walking; you will most likely see the lemurs. Allow at least three hours. But don't expect a pristine forest experience: the area is home to around 100 gold panners: you'll meet them and see the holes they've dug throughout the forest. It's fady (taboo) to harm the sifakas, so they are not afraid of humans.
You will need a 4WD to reach Daraina from Vohémar; it's not accessible during the rainy season.