It depends on how it was installed. Best way to find out is pull up along the edges and see. If you've got a transition strip nailed down, pry that up and then go at it with the floor.

Most vinyl flooring, other than that of recent years, is a multi-layered construction of a felt-like backing, a foam cushion middle layer and a design/wear layer over that. How old is your vinyl floor and what is its construction?

I don't know the exact age of the flooring, but I'd guess 15-20 yrs.

It is on concrete slab.

Tried a heatgun...didn't make much difference.
Scraped with a floor scraper which took of the plastic layer-leaving paper like layer. Ideas?

You really need to get it down to the slab. There is a tool...I think it's called "The Black Widow," that fits into a Sawzall (reciprocating saw) that looks a bit like a 4" or 6" chisel. Used properly, it really helps to dig up the old vinyl.

If you get it down to the cardboard-like material, sponge on a lot of hot water to loosen it. It should scrape up pretty easily after that.

My steam mop helps to loosen the paper/adhesive for easier scraping. But we still have quite a task ahead of us. The stuff should be outlawed :)

"The stuff should be outlawed"

If it is mineral fiber asbestos, it already is. If your floor is only 15 - 20 years old, it is unlikely that it has an asbestos felt backing.