Bear in mind I haven't used these, but from my knowledge of coatings technology, if someone is getting a soft surface just from thinning the paint, they are using the wrong reducing solvent, or doing something wrong when airbrushing, or did not prepare the surface properly.
Aaron Skinner of FSM, in his pull out from an issue date I don't recall, said that the recommended thinner was their Model Color Solvent, 70524. The Vallejo website says you can use distilled water—their instructions specify it. There is also an implication, at least, that alcohol may be used as a cosolvent (which would speed drying).
Aaron found no defects in the cured coating with respect to surface qualities or adhesion. He did say it required numerous thin coats to cover, but that's not necessarily a bad thing, especially considering that these are formulated to be thick in the bottle.
There is no such thing as a "paint hardener." There are accelerants that can be added to some paints, but they often change the performance of the paint in less than desirable ways.
I suggest you experiment and practice on some scrap before using it on a kit. I always do this with a new brand of paint, as well as test it for solvent compatibility.
Acrylics are NOT enamels, and CANNOT be treated or used the way enamels are.