welcome to HCM!
most common cause of such pustular eruptions in and around the vagina is bacterial infection,possibility of folliculitis
that is Infection of the hair follicle of the adjacent skin is there.it presents as a pus filled bump (pustular eruption) on the skin.iIn most of the cases, the folliculitis usually is self-limiting but sometimes enlarges and ruptures, discharging the pus. Folliculitis is generally treated with topical antibiotics and antibacterial wash two or three times a day. sometimes may require oral antibiotics
Few other conditions may present with lesions that may appear like pus filled bumps like genital acne
and certain cysts.
having such pustular,painful lesion of the vagina seems to be less likely but herpes
,needs to be ruled out.
You must consult a gynaecologist if the lesion enlarges further.
hope this helps you!