When my data (vertices, textures, et cetera) is purged from VRAM, how I can know that?
You can't. When you destroy your OpenGL resources (textures, VBO, etc) you are telling the graphics card that you don't need that resource anymore and it can do whatever it wants with the space they used; now, the driver can instantly delete them, mark them for purging or just mark the space as available for use. This implementation is transparent to the user and you can't know when or what happens.
Data in your VRAM won't get erased unless you explicitly call the glDelete* function. Therefore, OpenGL does not contain such notifications.
If you for some reason need such notifications, you'll need to wrap all glDelete* functions in your code into functions that call a notification routine.