VAT is a tax on consumption. However, while in some instances it taxes negative externalities (e.g. fossil energy consumption) in many other cases it taxes services without externalities or potentially positive externalities (e.g. advisory services on how to reduce carbon emissions).
If consumption is being taxed arguably it would make more sense to tax consumption that actually causes negative externalities. Carbon taxes would be an obvious candidate (though arguably not the only one). The price of a product in a value chain would increase as carbon is added to it, just as with VAT, only that the tax is on "carbon added" rather on "value added".
Presumably a carbon tax would also be easier to administer than a VAT since carbon is taxed only where it enters the economy?