Orthostatic Hypotension: This condition is characterized by a significant drop in blood pressure when standing up, which can lead to syncope. However, the presence of supine hypertension suggests a possible autonomic dysfunction, which can also cause orthostatic intolerance. The neck pain and headache could be related to the autonomic dysfunction or a separate issue.

Autonomic Dysfunction: This encompasses a range of disorders affecting the autonomic nervous system, which controls involuntary actions of the body. Symptoms can include orthostatic intolerance, supine hypertension, headaches, and vertigo, making it a plausible diagnosis given the patient's symptoms.
Cervicogenic Headache: This type of headache is caused by referred pain from the cervical spine. The neck pain and headache with vertigo could be indicative of a cervicogenic headache, especially if there is evidence of cervical spine dysfunction.
Vestibular Migraine: Characterized by vertigo, headache, and sometimes neck pain, vestibular migraine could be a consideration, especially if the patient has a history of migraines or if the vertigo and headache are episodic.

Pheochromocytoma: Although rare, this tumor of the adrenal gland can cause episodic hypertension, headaches, and even orthostatic hypotension due to its effect on blood pressure regulation. It's crucial to rule out this condition due to its potential for severe complications if left untreated.
Subclavian Steal Syndrome: This condition involves the theft of blood flow from the brain to the arm due to a stenosis or occlusion of the subclavian artery. It can cause vertigo, syncope, and arm fatigue, especially with exercise. Given the symptoms of vertigo and syncope, it's essential to consider vascular causes.
Multiple System Atrophy (MSA): A neurodegenerative disorder that can present with autonomic dysfunction (leading to orthostatic hypotension and supine hypertension), ataxia (which could cause vertigo), and parkinsonism. Early diagnosis is crucial for management.

Ehlers-Danlos Syndrome: A group of disorders that affect connective tissues supporting the skin, bones, blood vessels, and many other organs and tissues. Some types can lead to autonomic dysfunction, orthostatic intolerance, and joint hypermobility, which might cause neck pain and headaches.
Familial Dysautonomia (Riley-Day Syndrome): A rare genetic disorder that affects the development of the autonomic nervous system, leading to a wide range of symptoms including orthostatic hypotension, supine hypertension, and other autonomic crises. It's typically diagnosed in infancy but could be considered in the differential for unexplained autonomic symptoms.