I believe you would have to calculate the all the bone transforms and skin weights of the selected vertex and multiply manually.
This is because that multiplication happens in the shader (with GPU Skinning) when rendering so there is no way to access on CPU.
(If you turn off GPU Skinning, you may be able to access directly, but would incur the additional performance cost)
In Psuedo code,
Calculate which bones affect the desired vertex using the skin weights. (These are in the asset)
Get the ComponentSpace bone transforms from the RefSkeleton (these are cached every frame and reflect the animation)
Calculate the vertex postion.
Luckily, it looks like there might be a convenience function in MeshUtilities.cpp: CalcBoneVertInfos
(found in in Engine/Source/Developer/MeshUtilities)
It looks like this calculates all verts, so not sure if you want to run this every single frame…
but you could use this as a guide to roll your own “single vertex position calculater”.
If you do figure it out please post back - would be useful to have.