this is a crab-eating macaque (Macaca fascicularis), and it is an actual wild monkey rather than "free-ranging" (at least to me, that term implies it is a captive animal outside a cage, if that makes sense). The trees in the background appear to be mangroves, so here the macaques actually would be eating crabs*
*so much a better name than "long-tailed macaques"!!!
I'm not really sure, the place is operated as a kind of attraction with other animals in cages. Are they native to Vietnam? I don't know if they were introduced to this island.
they are native to the island, and indeed to much of southern Vietnam. Can Gio has an interesting history. During the Vietnam War the Americans destroyed huge areas of mangrove forest using defoliants like Agent Orange because the Viet Cong used them for shelter. The forests on Can Gio were pretty much entirely eradicated, and the mangroves there now are the result of extensive habitat restoration. The return of the forests has meant much better habitat for the macaques and they are now numerous (I have read totals of anywhere from "500" to "thousands").
