VAS Airplane Toolbox does not appear to be based on IRL A320NEO data. Its results are very inaccurate in many instances.
Currently, SimSmart appears to be the only one out there that is based on A320NEO data. Gavin acknowledges that it does have some accuracy issues due to the limitations of the dataset used in developing it. (There are also some other issues as well.) It is also payware, though the cost is low.
The upcoming A32NX calculator should be more accurate over the takeoff operating envelope of the airplane and will, of course, be free.