When to use VF layer?
Adhoc, by default, supports many different kind of charts. If you further want to add any new kind of chart which is not directly available, you can use VF. Also this VF can also be used by other people as well.
How to use VF layer?
Refer to the link to learn how to use VF layer.http://www.helicalinsight.com/technical-guide/how-to-add-vf-file/