Well to be honest it's not very different in concept at all.
However it's largely UK based, it's not just about food (anything vegan-friendly can feature, from campsites to shoe shops to local activist groups and one-off events/demos), and the emphasis is more on local community involvement than instant consumption.
And importantly for businesses looking to promote themselves, the Viva! name is well recognised and respected in the UK among thousands of vegans & vegetarians who've never heard of Happycow. There's no reason why both (and others, I'm sure) can't coexist.