On a quarterly basis we perform a disaster recovery exercise where we clone our PROD TFS environment by restoring the backups from a given date from our PROD TFS environment to another TFS 2015 environment. We've been doing this successfully with TFS 2012 and 2013. But when we attempted this with TFS 2015.1 the vNext build definitions are still getting source code from our PROD TFS server. We know how to make sure all of our XAML based build definitions are utilizing the DR environment but the vNext build definitions are a bit of a mystery. The system.collectionId GUID is different for the vNext build definitions in each environment. When we create new vNext build definitions they work as expected.
TFS 2015 Clone Process - vNext Build Definitions
To move vNext build definitions, the best way would be using the REST API to get the build definitions, modify the JSON file, and load the file back to create definitions in new projects.