Can transformer cooling oil form flammable vapour? Maybe, I guess, if it's hot enough, but I'm not sure. But when the transformer gets hot and explodes it may cause an oil spray fine enough to create a fireball, which may look similar. The first stage of a "proper" BLEVE is normally the "expanding vapor" cloud, which is visible as such, before it has mixed with air sufficiently (and/or reaches an ignition source) to burn and form the fireball. Then again, in smaller ones, and in the dark, the vapour cloud may be so short-lived that it cannot be seen.