Nicotine Stomatitis or Vaper's Mouth: Given the patient's history of vaping, this condition is a strong candidate. Nicotine stomatitis is an inflammatory condition of the oral mucosa that can cause redness, pain, and ulcers, particularly at the bottom of the mouth or on the lips. The patient's recent vaping history supports this diagnosis.

Aphthous Ulcer: Also known as canker sores, these are common, painful ulcers that can appear in the mouth, including under the tongue or on the lips. They can be triggered by stress, minor mouth injury, or certain foods.
Oral Trauma: The patient might have experienced minor trauma to the area, leading to the development of a sore. This could be due to accidental biting, dental work, or even the act of vaping if the device caused irritation.
Viral Infection (e.g., Herpes Simplex): Although less likely given the single lesion and the patient's age, viral infections can cause mouth sores. Herpes simplex virus can cause painful blisters or ulcers in the mouth.

Oral Cancer: Although rare and less likely in a patient without a significant history of tobacco use or other risk factors, oral cancer can present as a non-healing ulcer. Given the patient's age, it's crucial not to miss this diagnosis, even if it seems unlikely.
Infectious Diseases (e.g., Syphilis, Tuberculosis): Certain infectious diseases can cause oral ulcers. While these might be less common, they are critical to diagnose due to their implications for treatment and public health.
Autoimmune Disorders (e.g., Lichen Planus, Pemphigus): These conditions can cause oral lesions and ulcers. They are important to diagnose due to their potential for significant morbidity and the need for specific treatment.

Erythema Multiforme: A skin condition that can also affect the mucous membranes, causing ulcers and other lesions. It's often triggered by infections or medications.
Behçet's Disease: A rare disorder that causes blood vessel inflammation throughout the body, leading to symptoms like oral ulcers, among others.
Leukoplakia: A condition where thick, white patches or lesions develop on the tongue, the bottom of the mouth, or inside the cheeks, often linked to chronic irritation or tobacco use. While not typically painful, it can be a precursor to oral cancer.