I really enjoyed the food at Viva. The vegan selection was well marked and abundant, and I found the prices reasonable (I was there in the evening and heard it was cheaper then, towards the end of the day?). They had a big selection of buffet items, which you could mix - from individual salad ingredients to antipasti, spring rolls, ready-made salads, warm dishes, and desserts. You can put all you want on your plate und pay per weight of the food.
I enjoyed the taste and quality of the food. Maybe it was just the evening we were there, but people were rather quiet and the atmosphere pretty cold (which didn't bother us, as we were lively enough :-) ).
The only thing that bothered me as a vegan was that you could tell that non-vegan customers used tongs from the feta in the vegan antipasti dishes.
Cons: Not a cozy atmosphere