A child with a fever of 102°F and vesicles in the oral cavity is probably suffering from:
Acute herpetic gingivostomatitis

In some preschool children, the primary infection may be characterized by only one or two mild sores on the oral mucous membranes, which may be of little concern to the child or may go unnoticed by the parents.
In other children, the primary infection may be manifested by acute symptoms (acute herpetic gingivostomatitis). 
The active symptoms of acute disease can occur in children with clean mouths and healthy oral tissues. The symptoms of the disease develop suddenly and include, in addition to the fiery red gingival tissues, malaise, irritability, headache, and pain associated with the intake of food and liquids of acidic content. 
A characteristic oral finding in the acute primary disease is the presence of yellow or white liquid-filled vesicles.

Reference: McDONALD AND AVERY’S DENTISTRY for the CHILD and ADOLESCENT, 10th ed page no 245,246​
Herpes simplex type-I
Juvenile periodontitis
Acute herpetic gingivostomatitis
Neutropenia