I'm going to do the completely expected and disagree with gandalf about vanilla and CBM not being cross compatible. Personally I've played with CBM in almost every single game I've ever played in multiplayer. But, I've played a few games under vanilla, and was never unduly affected by the differences in them because there's not too much that shifts around that is important to remembering how to play. Basically anything that works in vanilla will work in CBM. Going backwards is a bit harder, because CBM makes a larger number of strategies viable than are so under vanilla. You can still use them, but they are not going to be as efficient So to a certain extent, it does make sense to get a decent grounding in vanilla Dominions. However...
A) This is not super important in this community, because nearly every game started runs the latest version of CBM.
B) Most games that do not use CBM are noob games, so it's not really a big deal if they're not completely cross compatible
You're not expected to do well here anyway!
And for the record, CBM stands for Conceptual Balance Mod, you can find it in the Scenarios, Maps and Mods subsection.