There are a few reasons your baby bearded dragon may be shaking, or "vibrating".
First off, you should make sure you have a UVB light/lamp, and make sure it works properly.
Perhaps your bearded dragon is getting cold, and is not able to digest it's food properly.
Bearded Dragons are cold-blooded animals, and require supplemental heat for proper digestion.
Also, I suggest getting a heating pad to place under the cage, they really help.
There is something called Metabolic Bone Disease (MBD).
Signs of metabolic bone disease are:
-Hard knobs in the long bones of the legs.
-Bumps along the vertebral column of the back and tail.
-Softening or hard swelling of the lower jaw.
-Jerky movements when walking
-Repeated tremors, twitches, or spasms in the limbs and muscles of the legs and toes when at rest or after exercise.
-Shakiness when being held.
If your bearded dragon has signs of MBD, I recommend taking it to the vet as soon as possible. There are several treatment options available for bearded dragons suffering from MBD.