Like the title says. I am planning to use Visual Studio AppCenter. Where does fastlane fit in there? As I am seeing it, fastlane becomes redunant when switching over to VSAC? Or am I missing the bigger picture here?
Visual Studio App Center certainly does a lot of the things fastlane does, but it depends fully on your individual use case if you can fully replace fastlane with App Center.
A major conceptual difference is that App Center is a hosted software as a service that you can use to do common things (build, test, publish) with your app. Fastlane on the other hand is a local tool (that can also be used in a server environment) that does whatever you want with your app. If there is no built-in action or plugin for your specific and custom use case, you can just write one yourself and add the functionality.
Fastlane is actually one of the installed tools on the App Center build servers, so you don't even have to decide if you want to use one or the other - you can still use fastlane with App Center.
(App Center also uses fastlane behind the scenes itself for features like App Store and TestFlight Distribution for example)