Hypertensive Encephalopathy: The patient's initial blood pressure of 220/120, accompanied by headache and vomiting, suggests a diagnosis of hypertensive encephalopathy. This condition occurs when severely high blood pressure leads to brain dysfunction. The reduction in blood pressure after 2 hours (to 170/100) could be due to initial treatment or spontaneous variation, but the symptoms and initial blood pressure readings strongly point towards this diagnosis.

Migraine with Aura: Although migraines can cause severe headaches and vomiting, the significantly elevated blood pressure is not typically a primary feature. However, some patients with migraines may experience blood pressure elevations during episodes.
Pheochromocytoma: This rare tumor of the adrenal gland can cause episodic hypertension, headaches, and vomiting due to the excessive release of catecholamines. The fluctuating blood pressure could be consistent with this diagnosis, but it is less common.
Acute Stress Reaction: Severe stress can lead to elevated blood pressure, headache, and gastrointestinal symptoms like vomiting. However, the degree of hypertension in this case is unusually high for a stress reaction alone.

Subarachnoid Hemorrhage (SAH): Although the blood pressure readings are more suggestive of a primary hypertensive crisis, SAH can cause sudden, severe headache (often described as "the worst headache of my life") and vomiting. The initial high blood pressure could be a response to the hemorrhage. Missing this diagnosis could be fatal.
Intracranial Hemorrhage: Similar to SAH, other types of intracranial hemorrhages (like intraparenchymal hemorrhage) can present with headache, vomiting, and elevated blood pressure. These conditions require immediate medical attention.
Posterior Reversible Encephalopathy Syndrome (PRES): This condition, associated with high blood pressure, can cause headache, vomiting, and visual disturbances. It is crucial to recognize PRES because it can lead to seizures and permanent brain damage if not promptly treated.

Reversible Cerebral Vasoconstriction Syndrome (RCVS): Characterized by sudden, severe headaches (often described as "thunderclap") and sometimes accompanied by vomiting, RCVS is a rare condition that can cause or be associated with elevated blood pressure. It is essential to consider this diagnosis due to its potential for severe complications, including stroke.
Autonomic Dysfunction: Certain disorders affecting the autonomic nervous system can lead to blood pressure fluctuations, headaches, and gastrointestinal symptoms. While less common, these conditions should be considered in the differential diagnosis, especially if other explanations are ruled out.