Couldthe medicine amitriptyline cause vivid dreams/nightmares?

Possibly. Although, usually it's just the opposite. Amitriptyline is what is known as a tricyclic antidepressant (tca). In general, tcas are known to inhibit or diminish rem sleep. Rem sleep is when dreaming occurs. A person may commonly have "rem rebound, " when stopping or reducing their tca medication then and dream more. Of course, there are always exceptions to the rule in the practice of medicine.