The injured girl was riding the the Hogwarts Express train with her family, who were on holiday from Tennessee visiting Universal Studios in Orlando.
Police described the explosion as a "fireball", burning the girl's face.
"It was just a weird, freak accident," Lieutenant Dan Brady told the Orlando Sentinel.
"It wasn't a big explosion but was definitely enough to scare everyone who was in that cab of the train," he added.
Her name has not been released.
The girl was riding the train, which connects Universal Studios with the Island of Adventure theme park, at about 15:30 local time (19:30 GMT) on Saturday when the device exploded.
The girl suffered mild burns to her face, arm, and leg, and was taken to hospital.
The vapouriser was inside another man's pocket when it malfunctioned, causing minor burns to his leg.
The explosion burned a hole in a seat, but otherwise there was no significant damage to the train, officials say.
Universal Studios shut down the ride temporarily to allow detectives to investigate.
The Wizarding World of Harry Potter opened at Universal Studios in 2010.

Meden Sports Centre in Warsop, Nottinghamshire, has faced ongoing maintenance issues with a temporary closures and Â£500,000 spent on repairs.
Mansfield District Council has said it is "unsustainable" and has launched a consultation on its future.
However, Warsop Parish Council said it was "essential" to the area and it would fight to keep it open.
The centre, which dates from the 1960s, was closed for six months in 2014 after an inspection identified fire safety concerns.
Officials have said renovation or rebuilding costs could cost between Â£1m to Â£8m.
Andrew Tristram, district council portfolio holder for the environment, said: "The council has undergone a review of leisure services at Meden Sports Centre and concluded that the current operating model is not sustainable for the future.
"There are increasing uncertainties about the running costs and capital investment due to the condition of the building".
However, parish council chairman Andy Wetton said: "The centre is essential to the area but this administration seems hell-bent on closing it.
"There was money available to replace it but this has been spent on other things.
"Any attempt to close it would be a declaration of war on Warsop and we would fight it hard."
The district council said no changes would be made before April 2018.

A judicial review has been launched after free taxis for pupils living 12 miles (19km) from Ysgol Brynhyfryd, Denbighshire, were stopped.
Parents claim their children's route to the bus stop is too dangerous to walk.
But a Denbighshire council spokeswoman said the policy was agreed after "extensive consultation" and through a "democratic process".
Glenda Coleman, who lives in Bryneglwys, said: "We've relied heavily on the free taxi because our children cannot walk to the bus pick up point as it is extremely dangerous.
"The council made a decision to cut this service without providing any alternatives or considering the repercussions of removing it."
Ms Coleman said it was "impossible" to drive down a nearby hill when it has snowed or there is ice on the road, meaning parents would not be able to transport their children to the bus stop, two-and-a-half-miles away.
She added: "We feel we are being punished by the council for living in a rural area as we have been provided with no alternative options for transport and it is unacceptable."
A spokeswoman for the council said: "Denbighshire's school transport policy has recently changed.
"We are aware that a small number of parents have raised concerns regarding these changes and their perceived impact.
"The policy was agreed after extensive consultation and it has gone through a democratic process.
"We have investigated complaints thoroughly and it is the right of individuals to consider taking the matter to judicial review, for which we would respond accordingly."
A 14-year-old girl suffered burns on a Harry Potter ride when another passenger's electronic cigarette exploded, police in Florida say.
[NEXT_CONCEPT]
Any move to close a sports centre would be a "declaration of war" on the local community, a councillor has warned.
[NEXT_CONCEPT]
Parents are taking legal action against a council following its decision to cut free school transport in a rural area.