First of all, keep in mind that I am unable to diagnose you because I am unable to examine you, this forum is for educational purposes.
The symptoms that you describe, headaches, neck/shoulder pain and dizziness are non-specific. These symptoms are often seen in cervicogenic migraine headaches (headaches that are triggered by neck problems). These type headaches are often refractory to medications, but do respond to neck physical therapy and/or botox injections. The MRI finding of a small cerebellar vermis (hypoplasia) can be related to a number of different conditions including a congential/genetic condition (such as a spinal cerebellar ataxia), long term use of anti-epileptic medications, and long term alcohol use. I would recommend that you see a neurologist that can obtain a good family history and based on that, may send some genetic tests to evaluate the cause of the cerebellar problem. The cerebellum is the part of the brain that controls balance and could be causing your dizziness (or may be unrelated). The term mega cisterna magna refers to an increased space around the cerebellum, likely related to the volume loss of the cerebellum.
I hope this has been helpful.