It depends. The typical stuff, that most people encounter, the Vietnamese "version" (called "nước mắm"), has a lighter flavor than the Thai "version" because, as was told to me by my mother-in-law who grew up in Vietnam, the Vietnamese prefer it to not smell or taste like fish. But, there's another type of fish sauce, that is Vietnamese, that is a little more rare. It's called "mắm nêm" and it has a very, very, _very_ strong fish odor and taste. I tend to steer clear of it but my wife and mother-in-law love the stuff.

edit/ For reference, I'm just an average white guy from the midwest and I married a Chinese-American woman and now live in SoCal. Through her, I've discovered, and love, most of these foods. Vietnamese fish sauce doesn't really register as "fish" sauce to me, either, rather it just tastes quite salty.