I was asked if there is a way of transforming configuration files at build time. Nothing else than the good, old and reliable SlowCheetah if you ask me
Just install the Extension, and you will be able to add your transformations by right clicking the .config file and selecting Add Transform:
These are going to be based off the BuildConfiguration you defined in your Solution. Once this is done you can define your own settings, like this:
where a transformation can be something like this:
There are so many examples on how to do this, so please do not shoot on the pianist 
Your Build Definition needs to specify a Configuration, either at Queue time or embedded into the Build Definition itself.
The result? As expected:
This isn’t really about VSTS or TFS per-se, but it is always a valuable approach to configuration management, and it was worth a refresher