You need to create simple media pool
with option 'Do not create, always continue current media set' on 'Media Set' step. Then create file to tape job
(since backup is a job-driven process in VBR job creation is required) pointed to that media pool and add your folder to the job. Every month you need to modify the job: add a renamed folder and remove an old one. The job may be set up to run automatically on step 'Full Backup' or may be started manually if you wish.