Probably a day-one purchase, and I'll certainly pre-order if there is a collector's edition or bonus associated with it.
By the way, Fallout: New Vegas isn't really quite Fallout 4. It's being developed by Obsidian (who are composed of several developers of the original games, including Chris Avellone), using Fallout 3's engine technology. You'd might call it a spin-off of Fallout 3, which many original fans consider to be a spin-off of the first two Fallout games. Yeah, fun. I'm expecting similar gameplay but with much, much better writing and role-playing elements (which, let's face it, are pretty much shit in Fallout 3).