You don't need permission if it is for personal use only. I'm not sure exactly what to do, but the easiest thing to do would probably be to copy the essex mod vehicles over the LA Mod ones, but you would need to change any files that refer to the LA Mod vehicles. If you did it the other way round, I think you would need to edit the scripts to correspond with the essex mod vehicles. I'm not sure which files would need changing exactly whichever way you were to do it.