Vasovagal Syncope or Near-Syncope: Given the sudden onset of hot flashes, nausea, and dry heaving lasting approximately 15 minutes, a vasovagal episode is a plausible explanation. These episodes can be triggered by various factors, including stress, pain, or even certain medical conditions, and are characterized by a sudden drop in heart rate and blood pressure, leading to reduced blood flow to the brain.

Anxiety or Panic Attack: The symptoms of hot flashes, nausea, and dry heaving could also be indicative of an anxiety or panic attack, especially in the context of a history of significant medical conditions that could provoke anxiety.
Gastrointestinal Issue: A gastrointestinal issue such as gastroesophageal reflux disease (GERD), gastritis, or even a viral gastroenteritis could present with nausea and dry heaving, though the hot flashes might be less commonly associated.
Medication Side Effect: Given the patient's history of Large B Cell Lymphoma, CVA, TIA, and DVT, they are likely on several medications. Some of these could potentially cause side effects like hot flashes, nausea, and dry heaving.

Cardiac Ischemia or Myocardial Infarction: Although the patient has normal vitals, cardiac ischemia or myocardial infarction can sometimes present atypically, especially in older adults. The history of CVA and TIA increases the risk of cardiac events.
Pulmonary Embolism: Given the patient's history of DVT, there is an increased risk of pulmonary embolism, which can present with sudden onset of symptoms like nausea and could potentially cause vasovagal symptoms due to the body's response to hypoxia or pain.
Intracranial Hemorrhage or Stroke: Although less likely with normal vitals, an intracranial hemorrhage or another stroke could present with sudden onset of nausea and vomiting, and the history of CVA and TIA makes this a critical diagnosis not to miss.

Carcinoid Syndrome: This is a rare condition that can cause flushing (which might be mistaken for hot flashes), nausea, and other symptoms due to a neuroendocrine tumor secreting serotonin.
Pheochromocytoma: A rare tumor of the adrenal gland that can cause episodic hypertension, tachycardia, sweating, and nausea, though it would be unusual for this to present for the first time in a 78-year-old without a history of hypertension.
Thyroid Storm: Although the patient's symptoms do not fully align with a thyroid storm (which typically includes fever, tachycardia, and altered mental status), it's a rare but potentially life-threatening condition that could be considered if there's a history of thyroid disease.