Some indications:

• Use of vertex group is not functional
• This is the same for simple deform modifier which was already ported (so I think there is some external causes to this issue)

See "modifier_get_vgroup_mesh" calls in line 280 for hook and in line 285 for simple deform.

The resulting deform verts is null because the input mesh is null.

In the previous code for the hook, it seems there is a workaround for it, which is before calling the modification function (I was not able to find an equivalent workaround for simple deform in the history, so I presume it is preferable that things works without it):

	DerivedMesh *dm = derivedData;
	/* We need a valid dm for meshes when a vgroup is set... */
	if (!dm && ctx->object->type == OB_MESH && hmd->name[0] != '\0')
		dm = get_dm(ctx->object, editData, dm, NULL, false, false);

        deformVerts_do(hmd, ctx->object, dm, vertexCos, numVerts);

I've try to setup an equivalent workaround for Mesh, but I failed as Blender crashes after the call to the modifier, when the resulting vertexCos are reported to the mesh (access violation).

To reproduce the issue, in both cases (hook and simple deform), simply create a vertex group, assign some vertices to it, and check the result when the vg is used in the modifier.