bulbapedia.bulbagarden.net/wiki/Individual_values
a quick search of the internet and multiple sites from multiple resources will show you the information you are looking for. I would love to run down the basics of IV to you but I'm sure you can read and besides, I might say something that is incorrect and get called out on it (done this multiple times).
I believe Vitamin items only increase EV's and only to a certain threshold.