The Vault recently underwent a long quiet period with no content being posted. This led to concern in the community about the future of the Vault. To avoid repeating the whole story, you can read more in this Bioware Social thread.
The point I'd make here is the poster DarkOne is the owner of the Nexus group of websites, and has been willing to work with the community, both in that thread and via the NWN irc channel. Funnily enough, it's almost one year exactly since the gmod.com debacle, where that site simply took people's content without asking permission.
So DarkOne and the Nexus are providing the community with a Neverwinter Nexus site.
If you play the Elder Scrolls games, or the Fallout series, you are probably aware of the Nexus. The Nexus has been probably the primary community place for mods for those games.
You can see my content on the Nexus. I put it there. The Nexus has some good things, like no 25 meg file limit. You can post any size files you want without having to wait for approval. I'm hoping the Neverwinter Nexus draws in new players, people who like the other Nexus games and are made aware of all the content NWN has when they see a NWN Nexus. I'm also hoping we see some cross pollination of modders, with people that mod those other games getting interested in modding, or porting their existing mods, to the NWN series. My content will still go on the Vault as well, I will just be posting it in both places.