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Do you mean using PVA as a bonding agent? If that is the case then SBR is a better option however in this instance given the area is so small then partially bonded should be ok.
I've laid a fair few screeds in both domestic and commercial environments and never heard of mixing a screed with PVA.
Bobbo-
I mean to avoid the inevitable shrinkage cracking from a simple sand/cement mix.
The Fosroc screed and grouts you mentioned upthread typically had additives to ensure dual phase shrinkage compensation.No need to add anything to a bag/sack of a branded screed or grout,
but for this application replacement of some/all of the water with PVA,
(commercially available cans will be dilutions of PVA in water),
will improve the workability and should will the risk of shrinkage cracking.
mespilus
If the water is replaced by PVA you might be able to avoid shrinkage cracking as the sand/cement mix goes off.