Sorry to see nobody has responded on your question yet.
Unless snoring is causes by cold symptoms I would say a vaporiser wouldn’t help.
How persistent is the snoring ? And does it disturb her sleep pattern ?
The most common cause for snoring by children is large or swollen tonsils and adenoids: The tonsils and adenoids are found near the back of the throat, and they are part of the body’s immune system. If they are naturally larger or are swollen because of infection, the tonsils and adenoids can obstruct the airway and cause snoring. This is the most common cause of sleep-disordered breathing in children.
A vist to the Gp may be good. Your Gp may refer you to an ENT specialist and sleep study may be done.