That's what I'm trying to tell you, every plugin that saves data on map end will fail to work sometimes. nVault should appear in your modules - it does in my server at least. Try removing all plugins leaving only the rank system and see if the problem persists. If it does, reinstall the entire server and AMXX.
Also, If i'm correct, you won't have this problem only with nVault, but with every plugin that saves data on map end or player disconnect. The only difference is that nVault works in a way that if not handled properly it can reset the data, while some other methods will lose only the data gained on that current map. So, using another method is not a solution either, because you will still use data.