EEE-URRR
Wrong. 'Viking' metal...well, has to be about Vikings. It's probably the only criteria for calling something viking metal, because otherwise the bands that you might class as viking metal are spread rather wide. Your standard viking metal is your Bathory-a-like: combination of clean and weak black metal vocals, choirs (often all male), songs may prominently include acoustic guitar, so on so forth. But still, I would certainly call Amon Amarth and Ancient Rites Viking metal, and they don't sound like that, or each other. It's all attitude. There's also quite a lot of subgenres that make you think maybe 'barbarian metal' might be a better sobriquet: You've got bands like Cruachan and Waylander with their pipes and whistles singing about celtic fury and killing vikings, Forefather singing about Anglo-Saxons clobbering Celts, Korpiklaani, Finntroll and Kalmah singing about trolls, goblins, swamp-men and goodness knows what kicking the everliving shit out of scandinavian christians, and so on and so forth. Ancient Rites even managed to write a few songs about the historical achievments and victories of the Belgians. A lot of it probably teeters on the brink of racism for anyone with a politically correct view about what pasty caucasians are allowed to say, but that's really not what it's about at all. Viking metal, and everything related to it, is 2 parts historical revisionism, 2 parts a celebration of the bands own culture (or another culture they think is really neat) and 2 parts balls to the walls.
Folk metal is something different again, though they are clearly related and often cross over.