FREE. You should not pay anything. Even if you did not have a warranty, there are implied warranties of merchantability and implied warranties of fitness for a particular purpose unless items are sold "as-is." So, unless you signed something that said that you accepted responsibility for cracking due to an old house settling, or otherwise signed something absolving them of liability, someone's got to get you a new counter at no cost to you!

I hope you have photos of the crack from right after the installation.

I am so sorry that this has happened to you. I recently had an issue with our floor and when I contacted the mfr, they kept saying the installer was at fault and the supplier. In the end, the supplier is refunding material costs and the mfr is covering the labor. I'd try for a new slab from the vendor and new fabrication from the fabricator. Since both probably want to work with each other in the future, they should play nice and split the compensation that way. The vendor blames the fabricator and the fabricator will probably blame the vendor. In my case, I told the mfr to work it out amongst themselves and let me know when they had worked out a deal, which they ended up doing.
I'd ask them to meet with you at the same time and come up with a plan. Both may be less inclined to blame the other to their face. You may be stuck with the tile issue as neither one will will to pay out of pocket for that. That may need to be your contribution to the fix.
Good luck and stick to your guns! Both will try to get out of it, so you need to be firm and make it clear that you are the only one who is ,not at fault. If your floors and walls did not settle then how did the counter? Older houses usually have settled most of the way in the early years (unless you have a serious structural problem) and settle much less after the initial phase. If you had a gc on your kitchen, I'd ask for their opinion as to settling and/or why it is cracked. An unbiased outsider may have more insight.

Did you have a GC? Who was responsible for making sure the floor was even and the cabinets stable?

Hmmm, I'm with warmfridge. I think that whoever was responsible for making sure the substrate was solid (floor and foundation strong enough, cabinets well secured and perfectly level, etc) should probably be atleast mostly responsible for the costs. Who was that?

If you have an older house, it seems doubtful that the cause was "settling" unless you made major structural changes like removing load bearing walls or adding a huge amount of new weight to a floor not structurally capable of supporting it. As dianolo said, houses settle most right after they're built, and then only very gradually afterwards, usually sinking slightly towards a central chimney, if there is one. But I wouldn't think drastically enough to crack a stone counter, unless structural changes were made or it's built on really unstable ground, in which case you'd be seeing ongoing plaster cracking as the house moves.

who bought the material? that makes a difference.

if you bought the material and gave it to the fabricator to fabricate, that can cause and issue. if the fabricator purchased the material then there that is a different story.

the problem when you purchase the material and the fabricator fabricates it is, if the material is faulty, its not the fabricators problem, its the problem of the person who supplied the material, you. then you have to deal with the person who you purchased the material from.

same problem if the fabricator installed it improperly. it breaks, ultimately fault ends up with the fabricator, but the material supplier would be saying that their material is fine, and not refund it.

in a situation where you aren't in the middle, the fabricator would have to fix on his dime. essentially repurchasing the material and refabricating

Thanks for responding, guys. Oh lord it's getting complicated! Sorry. Feeling a bit overwhelmed. I should be relieved and happy the kitchen is done, not feeling like I'm about to hit myself over the head with a 2x4 because my new counter is cracking up. It's cracking up. I'm cracking up.

It's a good idea to bring in the GC. Considering how hard it is to bring in one of these guys, getting all three together sounds about as likely as a Christopher Peacock collection at IKEA.

The fabricator purchased the slab though we selected it. I'm uncomfy with the onus being entirely on him. I'm fairly sure that this slab is faulty because in addition to cracks where cabinets meet, there are also random cracks in the middle of cabinets. Nothing to do with settling at all. We moved one window and cut an entryway into one wall (across the room from the counters). There were no huge structural changes.

The GC installed the cabinets. However, the countertop fab guy was not one of the GC's subs, but our own found through GW. We'd read so much here about the importance of getting a fabricator who's familiar with soapstone so that's what we did.

My head hurts. Sorry to be dense, but I'm still not sure what's reasonable to ask for. I want a goal to work toward before speaking to these guys. If I don't have a concrete goal in mind, I'm worried that we'll be swayed by all the blather.