Van Long looks like the photo below by @Maguari. Imagine that all the outcrops are surrounded by wetlands (i.e. not just at the front as it looks from the photo), and that is really the whole reason the langurs still survive there.
@Chlidonias I realize I never responded to your question. Looking back at my observations from then now, my boat tour started at around 14:30, I saw the langurs at about 16:00 and the boat came back at around 16:30. It's crazy that I spent 2 hours there, it felt so much shorter than that!
I suppose your theory of them coming out in mornings and late afternoons is probably correct, that makes a lot of sense.
