Assuming you loaded the FBX through the content pipeline, it would have assigned it one of the frameworks standard vertex types. But there is no vertex type that has only position and normal information. Most likely there is a 3rd element that is screwing up your results. This should work:
int vertexStride = model.Meshes[0].MeshParts[0].VertexDeclaration.VertexStride;
VertexBuffer vb = model.Meshes[0].MeshParts[0].VertexBuffer;
List<Vector3> vertexPositions = new List<Vector3>();
for(int i = 0; i < vb.VertexCount; i++)
{
Vector3 vec;
vb.GetData<Vector3>(i*vertexStride, vec, i, 1, vertexStride);//3rd param should either be i or 0
vertexPositions.Add(vec);
}