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An acute febrile rash accompanied by vesicular lesions in a young adult is most commonly associated with infection by the Varicella zoster virus, which causes chickenpox. This viral infection is characterized by the sudden onset of fever, followed by the development of a pruritic rash that evolves from macules to vesicles, and then to crusts. The presence of febrile symptoms alongside vesicular lesions strongly suggests a viral etiology, particularly Varicella, which is known for these systematic manifestations.

In contrast, other conditions such as contact dermatitis typically present with localized areas of irritation and do not involve systemic febrile symptoms or vesicular lesions. Impetigo, while it can cause vesicles, generally arises as a bacterial skin infection characterized by crusted lesions and does not typically present with fever early on. Pemphigus is an autoimmune blistering disorder that may cause vesicular lesions but is usually not associated with fever as a prominent symptom. Thus, the combination of fever and vesicular lesions points decisively to Varicella zoster virus as the underlying cause.