What does 'diffuse severe attenuation of the left vertebral artery" mean on a neck ct scan with contrast? could it be cause of previous spontaneous arterial brachial clot or subclavian clot?

Hard to: say with certainty without access to the images and report, but most commonly this is simply due to a developmental/congenital anatomic variation whereby one vertebral artery is very small. This would not be the cause of a clot elsewhere.

Arteritis?: If the person with this also had previous arterial clots (subclavian, brachial, or any artery) that were spontaneous, i.e. not following trauma, surgery, or other obvious cause, it suggests a problem causing generalized arterial inflammation (arteritis). As in my other reply, involvement of the vertebral artery implies risk of stroke. This person urgently needs expert medical care.