See if you can delete some of your older backups and give enough maintenance window for the garbage to clean up.
Note -
If you are using VDP 5.1, then increase the blackout window temporarily - For garbage collection (cleanup deleted backups from storage)
If you are using VDP 5.5, then just increase the maintenance window temporarily.
On longer term you may have to rework on the retention policies so that you'll not end up with too many backups.