A vesicle is a small fluid-filled blister. A vesicular rash is a scattered or linear distribution of sharply circumscribed, blister-like lesions, which are full of clear, cloudy, or bloody fluid. The lesions are usually less than 0.5 cm in diameter and may occur singly or in groups. A vesicular rash may be mild or severe and temporary or permanent; it may result from infection, inflammation, or allergic reactions. Usually vesicles break easily and release their fluid onto the skin. After this fluid dries, yellow crusts may remain on the skin surface.

Burns : Thermal burns that affect the epidermis and part of the dermis cause vesicles.

Dermatitis With contact dermatitis, a hypersensitivity reaction generates an eruption of small vesicles surrounded by redness and marked edema.

Erythema multiforme is heralded by a sudden eruption of erythematous macules, papules and, sometimes vesicles. The typical rash appears symmetrically over the hands, arms, feet, legs, face, and neck

Herpes simplex produces groups of vesicles on an inflamed base, most commonly on the lips and lower face.

Herpes zoster, a vesicular rash is preceded by erythema and occasionally, by a nodular skin eruption and unilateral, sharp, pain along a dermatome.

With insect bites, vesicles appear on red hivelike papules and may become hemorrhagic

Tinea pedis causes vesicles and scaling between the toes and, possibly, scaling over the entire sole

Smallpox after two three days of small pox the boils turn into vesicular and then pustular.

Other reasons that can cause Vesicles are:

Allergic reactions to drugs
Atopic dermatitis (eczema)
Blistering skin diseases including porphyria cutanea tarda and dermatitis herpetiformis
Chicken pox
Impetigo