I've heard there is a nerve in the windpipe that can be injured or damaged by the vent. Also, a person can develop a granuloma from the ventilator.
Granulomas can be serious. And they can become emergent situations.
My son had a serious episode with a granuloma after being on a vent -- his voice was "gravely" -- and the granuloma had to be emergently surgically removed after it suddenly flipped up and cut off his airway. His voice was immediately normal after the surgery and the recovery afterward was very easy for him.
(I'll add an edit here, hoping more details might be helpful: Our son actually had developed two growths -- a smaller growth on his vocal chord, which another ENT saw with a scope and didn't deem dangerous, but that specialist didn't drop the scope down far enough, and missed seeing the larger growth on his windpipe, which is what emergently manifested itself in such a dangerous situation several weeks later while he was sleeping.)
You may need to see an excellent ENT (ear, nose, and throat) surgeon; I'll try to spell their special name, ootolaryngologist, or something close to that.
Best wishes and please let us know how things go.