Solid Hardwood, Engineered and Laminate Flooring - Leveling before laminate tiles

I'm planning on installing laminate tiles. Existing floor is vinyl. Most of the area is nice and flat, but an 10'x8' alcove was added from the old wall, outward from the house, over a slab.

From the old exterior wall to the new alcove exterior, there is a gradual drop to a full 1" lower. It's less on the two perpendicular sides, but dished to a full inch lower at the middle.

I've read a couple times that 30lb roofing felt is used, but is an inch quite a bit to expect from that? It sounds simple for a DYI job, and it'd be easy to incrementally feather it up to the original grade.

Would the felt be too soft? Is an inch too much?
Alternatively, if I use a leveling compound, can I place it over the vinyl or does the vinyl need to come up? Thanx.

Martyr

I just completed a floor leveling project in my home and it's quite simple. Just use one of the many self leveling cement products available now (quikrete, ardex, etc.) here's a link to quikrete:
http://www.quikrete.com/diy/project_65.html

You need to pull the existing vinyl up first. A heat gun really helps. Then just mix the cement up in a five gallon bucket using a half inch drive electric (not cordless) drill and pour it out. You might want to transfer the cement to a smaller bucket so you can control it better.

Good practical tips. Thanks.

If I were called on to inspect a laminate flooring warrantee claim and found roofing felt was used to "level" (or in fact make flat) I would reject the claim.