I am a little confused, I can't really visualize where you are going to put the compound. I have done quite a few fireplaces, so I am speaking from experience...
I would not use a vinyl spackle compound near heat. If you are going to DIY, I would use a non plastic compound. I only use slaked lime plasters on fireplaces, so I can't even recommend an american product. If I were you, I would check out the ingredients in a basic joint compound and hit the sample boards again. A good source for MSDS info is the USG website. Another alternative would be to find an applicator from this website that is near you. If you want a true Italian look, you found it. Good luck.

Click on this link
http://www.italianplasters.com/photopost/showphoto.php?photo=70&password=0&sort=1&cat=500&page=4

This was a fireplace that I built out of plywood. The reason I had to make it out of plywood is because the height and width exceeded your mormal cut of 3/4 inch board, whice runs 4x4, or 4x8. I needed a height of 5.5'.
So I used plywood so I could cut out the arch, mitre the arch to round it out, and mainly because I had to use a biscuit joiner to bring the two pieces together. I then primed the wood with oil, sanded it, joined them, attached then and taped and mudded them like it was drywall. Worked nice. I surrounded it with Wenge wood,..100 bucks a board.

Good job Amy Scott should appreciate your craftsmanship
and design, wonder what Scott would have came up with?
Thats some work now.

I love its simplicity and elegant style, sleeeek...

The first post I made on your fireplace I thought Amy was
responding to your post when she was answering the previous question on the spackling, anyway that post did not make any
sense after looking back.
I am looking at finishing out the cast stone
put in my house, I am thinking about doing it in Aquarello
right over the cast stone only layering it at 50% to incorporate a few different colors.

Anyway, that is very nice, and the color is awesome.

Vallie