A VCL (or foil face used as a VCL) is used in a timber structure (wall, floor ceiling, roof) to stop water vapour getting through to the timber. So its not a requirement in a concrete floor.
If you were pouring wet concrete, then a polythene layer (or taped joint) may be useful to stop water dripping through joints and lifting the insulation. But as you will be using dry screed, such precautions are not required for this.
Recently a builder told me that a building inspector had said they have found cementitious screed eats PIR and so a polythene separating layer is needed.
Ive also heard that some building inspectors like to see a plastic sheet between insulation and screed to act as a 'slip layer' with underfloor heating.
I cant say Ive heard any official confirmation so feel free to ignore!
Dont forget to use perimeter expansion foam between screed and walls......
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