They may begin selling ticket in about a week (Aug 25) for flights begining Oct 2. They will start with a few flights and add new ones later.
Most initial flights will be between Puerto Rico and the DR.
See
http://www.vivaairlines.com/schedule.htm
for the proposed initial routes.
I don't know when they will fly from the US but Falcon Air is sub-contracted to service some flights according to:
http://www.vivaairlines.com/destinations.htm
My guess is that they will service initial flights originating from Fort Lauderdale (don't quote me on this).
You could call their 800 number next week when it is activated (800) 505-VIVA or keep an eye at:
http://www.vivaairlines.com/planatrip.htm