Here is your problem: "This leveler was put on top of a few layers of old viny..."

A few LAYERS of VINYL....wt???? So. This is why a professional tile setter worth their SALT will NOT allow install over ONE layer of vinyl let alone "a few layers". They should have made you sign a waiver saying they aren't responsible for the install - or better yet they would have told you "No. Sorry Ma'am. Not going to happen. It comes out or walk out."

It sounds like several things have happened. First, the vinyl layers (plural) were not removed (super duper wrong move). Second a "deflection rating" was NOT done on the UPPER LEVEL of a WOOD FRAME home (technical but again a super wrong move on everyone's part). And the straw for the other camel = no one CORRECTED the subfloor deflection rating to allow TILE!!!

Tile is HEAVY. So heavy that a wood framed building MUST have specific deflection ratings (joist spacing + thickness of subfloor) before tile can be installed. The fact that your self leveling is CRACKING before the tile is set, tells me the floor is too heavy for the frame. That means the cement poured is TOO HEAVY! Let alone the other 300 lbs you will add to the floor with tiles, mortar and grout!

There is a reason why there are several layers of vinyl. It is one of the few floors this structure can handle. I suggest you stop what you are doing, remove everything and start again. This floor is too heavy for your structure. And the tile isn't even in yet.