Never make changes to the vanilla files in the vanilla folders. They'll just be overwritten the next time Paradox issues a patch. Always make a mod to make your changes. You just need to make a mod file and a folder to place your modded files in... in the appropriate place in the directory tree, of course, so CK2 can find them.
You'll need different 00_cultures.txt files for vanilla+BLC, and CK2Plus+BLC. Vanilla's cultures aren't CK2Plus's cultures aren't Lux Invicta's cultures aren't HIP's cultures. Have you considered BLG as a more accessible alternative?
Thanks for the advice. I'll go back and undo my changes. I am not familiar with BLG.