Anchor on two words: painful and vesicles. Among genital ulcer diseases, the lesion that is vesicular before it ulcerates and is painful is herpes genitalis, driven by herpes simplex virus (predominantly HSV-2). A primary episode also brings fever, malaise and tender groin nodes, matching the systemic picture given.
Contrast this with the other classic genital infections. Chancroid (Haemophilus ducreyi) is painful but presents as ragged ulcers, not vesicles. HPV gives painless cauliflower-like warts. Syphilis (Treponema pallidum) gives a single painless, firm chancre with painless lymphadenopathy. Only HSV unites grouped vesicles, pain, and constitutional symptoms.
\[\boxed{\text{Herpes simplex virus (HSV)}}\]