Viera are not even present in Final Fantasy Tactics (both PS1 and PSP version). At that point in Ivalice there are probably hiding from all the conflicts, as probably most of the races also do. I'm not 100% on that though. I don't believe you learn about their race in the other 2 Tactics titles and Vagrant story, although I never played, I do believe no Viera are present.

I hope the next Ivalice title explains a lot more about the world of Ivalice.