Common for Virtual Form control and Virtual Form Designer is an *.VF file in which are stored the definitions created from Virtual Form Designer and used by Virtual Form control.
In this *.VF file are definitions that Virtual Form control uses for building Virtual Forms (fields for Grid control and Edit control, command buttons used on specific Virtual Form, relationships used to bind master and detail sections of the Virtual Forms, events that we want to be fired by Virtual Form control).
We can have multiple Virtual Forms controls in the same project and every Virtual Form control needs to have a VFFile.
If we change the filename and path of the VFFile, we need to change it also in the property VFFile of the virtual form control.
About where to find and where to put the VFFile you can read in this documentation article.