Do you have a specific modding goal in mind or would you like more information on getting started with GTA V modding?
The files you mentioned, x64e.rpf and vehicles.rpf , are part of the game's data files that contain level and vehicle information. Here's a brief rundown on what they are and how mods interact with them: gta v mods x64e.rpf levels gta5 vehicles.rpf
You're looking to mod GTA V!