Authenticate against an external database.
 

Hey. I'd like to know how I'd go about authenticating against a third party database and/or the vbulletin one

E.G The user enters a username/password on my vbulletin,

Step 1: Check if a login exists @ vbulletin.
Step 1a: If so authenticate the user as usual.
Step 1b: If no send details to third party and request authentication, third party will respond with valid/not valid message.

Does anything like that exist and if not where would I start researching that sort of setup?