A reaction from a chicken pox vaccine? If it's a reaction then I'd just keep a close watch for other symptoms like fever, vomiting, etc. I have an allergy to ampicillin and break out in a bright red itchy rash, but it went away on it's own. If the spots are causing itching or burning or something like that ... then stuff like coconut oil, aloe vera, etc., might soothe it.
If it IS the chicken pox, it will run it's course and immunize your child from the chicken pox.