Trying to figure out a way to redirect the https://naX.salesforce.com/secur/login_portal.jsp page to a Custom VF login page.
Here is my use case:
My org is creating a new Customer Portal with custom VF pages replacing all the Standard SalesForce pages for each tab. We will be transitioning all portal users to new Portal shortly. For users that have bookmarked the Current standard salesforce login page we want to redirect to our new VF login page.
Best Answer
You could set up a Site which would front-end your customer portal and direct all of your users there now. There is no mechanism to automatically direct someone from the Portal login page into a Site login page (or anywhere else, really).
At the point where you will perform the cut-over from one customer portal to another, you would only need to alter the Login Settings configuration on the Site to use your 'new' portal as it's authentication provider.
Your authenticated users would then see the 'new' portal via the same Site that they've been using.