I am not sure the Congress of Vienna was "fully" within "international law"; it was a multi-lateral treaty. Consider, the treatment of Poland under the treaty. In a way, it was similar to the earlier partitions of Poland (1772, 1793, 1795) based on multi-lateral treaties (just fewer signatories). If only taking the Polish angle, some aspect of a true international law seem missing from Vienna.
For the most part there was no established and comprehensive "international law". Instead, there existed "observations", "concepts" or "principles" suggested by several authors (Belli, Grotius, van Bynkershoek, Burlamaqui and others among whom one could include the Pope, if one wishes). Then, there were the several bi-lateral or multi-lateral treaties (again, falling shy of a true "international law").
The references in other posts to Martens and Schoell primarily contain documentation of the various treaties and, for Martens, his essay on the theory for laws of nationa and guidance as to diplomatic matters.
So, when considering the actions of Napoleon, absent specific agreements care is required before making absolute assertions of lawfulness or unlawfulness.
One could argue (as the Emperor did) that the many changes within Europe during the Empire fall within "international law" as they arose out of treaties. Naturally, if they did not meet a wide enough acceptance, that would not stop Napoleon. Still, he managed to get enough parties to sign for his taste. This may have been too limited and excluded "decenting" nations or "families" who had a counter claim or, at least, found the action inconvenient.
The accepted norm was rather open in the era, as few nations were free of such "outside the law" behavior; just consider the Russians in Poland in 1792, the Prussians in Hanover in 1805 or Britain in Denmark in 1807. - R