Flooring latex breaking up under vinyl

Hello, in the beginning of the year, we had a large area laid with vinyl. A self levelling latex was applied to the concrete, and the vinyl laid over that- I understand this is common practise. In some areas the latex has crumbled and the floor is no longer level in many sections. I don’t believe I have waterproof membrane below our floor due to the age of the property. We have no other signs of damp on the walls or any musty smells. A damp tester has been used on the areas where the latex has crumbled and some damp has been revealed. The upheaval and cost of digging up the concrete and fitting a membrane is not a solution I would like to consider. Lifting up the vinyl, removing the compromised latex and coating the concrete with a waterproof paint maybe an option, but I understand it may not be a long term solution. It is obvious that the latex does not like the damp, is there and alternative underlay that won’t react to the damp? Normal carpet underlay?