The devs have kept it very vague about the ages of the companions, probably on purpose. My guess, like Kain's, is about mid 30s for Vivi.
That does tie into why I love this series. Many of the companions and heroes are in their mid 20s-late 30s (except for Wynne, which I put around 60,) and it's so nice. I played JRPGs for years, and I still do. I enjoy them, but I do get tired of the "teenager saves the world" trope that JRPGs (and a lot of anime) have driven into the ground.
It has been only ~3 years since the end of DA2; Varric's interogation took place in 9:37 Dragon. The events of the entire game (from the very start til the interrogation) is over a 10 year time span.
As far as in Inquisition, I think we're at 9:41 Dragon at this point, as Asunder takes place in late fall of 9:40.