First of all, make sure that your redirect URI is correctly specified under your Application settings under your Layervault Account settings.
Then, when configuring your oAuth 2 client library, tell it to use https://api.layervault.com/oauth/authorize to request authorization and https://api.layervault.com/oauth/token to get access tokens. Most libraries will default to this convention anyway.
If you're building a Rails application and using OmniAuth, we have a pre-baked Authentication Strategy for you here.