I'm confused also as there isn't normally an index.html in src > theme > adapt-contrib-vanilla !

There's one in the framework's src folder: https://github.com/adaptlearning/adapt_framework/blob/master/src/index.html

But if you are using the spoor plugin it will be overrwritten by this one. Additionally the spoor plugin adds an extra launch page which only gets used when the course is launched from an LMS.