If you are looking to configure the claims based enterprise portal with Azure ACS you might find these sequence of posts worth working through as there is a reasonable amount of fiddly parameters to configure and new concepts to understand. Here is the sequence of posts to work through.
This part covers a few concepts and some functional background behind what you will be doing.
This part covers the initial creation of the anonymous portal site.
This part covers the local authentication settings that you need to configure to allow anonymous user and the guest user to login.
This parts walks through creating the Azure ACS namespace and connecting it to your public site.
This parts walk through the handling of the identification information sent back from Azure ACS.
This part covers the configuration of the Azure ACS site and the public site to handle the login what will be used in the authentication step.
This part is the last step which covers the hand over from the public site to the private site.
At the end of this sequence then you will have and environment that you can use to accept claims based logins either via the vendor portal or the customer portal or what ever functional you want to display via the security roles you assign to the user.