Yes definitely want to keep the Vault independent. Plus moving the vault to steam means everyone would be required to have that version - as you're probably aware not everyone likes steam. Stuff on the vault can be used by all versions of the game whereas steam workshop wouldn't (technically you could transfer files manually between installs but you still need the game on steam to download from it's associated workshop).
I've kept my BD client version for now, mainly because it's a bit easier to manage the files compared to how steam's workshop sets things up for you. But I tinker a lot and have been mucking around with NWN for a long time. If you want things simple then steam is a good option, provided people start uploading their work. Being old I imagine there'll be a lot of stuff that never makes it on to steam.