A couple of weeks ago a story was run that Terraria on Vita would release on December 4th, which turned out to be inaccurate as the date passed without the game showing up. However we now have actual confirmation of a solid date from the developers themselves.
Terraria will now release on Vita on December 11th in Europe, while North America can get down to exploring and digging on December 17th. There is also an update live for PS3 owners that adds Cross Play to the title ahead of the Vita release.
Source: Terraria Online