It could be a few things really, however its not unheard of to get a faulty or imbalanced clutch as manufacturing faults do occur.
The release bearing is always a likely culprit as these do suffer from rough/worn bearings which would give a vibration, but you'd normally get a rough feel through the pedal too.
It's also possible that there is an issue with the pressure plate, although only an issue with say several broken springs etc would cause the vibration you describe, in this case the clutch would feel different too. Basically If anything has broken off a pressure plate or cover it would cause an imbalance in the assembly.
A damaged or worn spigot bearing is also a possibility as when the clutch is disengaged this basically keeps the input shaft central. If the bearing is damaged or collapsed the input shaft is just left to float around in the flywheel, and as the clutch plate is almost on the end of the spigot shaft vibration would be amplified and felt through the pedal.
Basically if there is vibration that shouldn't be there in the clutch assembly it should be checked out. As vibration in a faulty/damaged rotating assembly won't usually get better, it's normally the opposite.
Depending on how bad the vibration is.. How long you leave it is up to you I'm afraid, all I can advise is that you don't ignore it, especially if it is becomes or is becoming worse.