External url for a vf page

HI I am implementing oAuth to a website from salesforce.So the use case is that when user clicks a custom button it opens a pop up window.Then user authorizes the access from this window and it is closed after the authorization is successful.Then apex has to store access_token in salesforce.

The problem is that I have to give a url to which the target site should redirect with a access_token after a successful authorization.So I am thinking of giving the url of a vf page which on load can check the access_token in query string and save it to salesforce,but what should be the external url of the vf page?

I am open to other suggetions as well,please suggest any other way by which I can do the oAuth from salesforce.

Thanks