I don’t think they are used interchangeably. I think I would consider something very interesting if I had never heard of it before. I consider something to be extremely fascinating if I can hardly tear myself away from hearing about it or seeing it. I found reading about the Northern Lights (Aurora Borealis) to be very interesting but I could have watched them all night long as they were extremely fascinating.

You can use them interchangeably. I would say that if I was reading the two, I would be inclined to believe extremely fascinating is even more interesting than the idea of “very interesting”. If that makes any sense. :)