Yes, you can do the old .vob replacement. But can you play multiplayer with .vob replacement mods and see everyone with different modded car? I don't think so
To be fully honest, there are not very good mods at the moment, maybe an year or some more time, things will be different. Anyway, the tool - LFS Editor is worth the price of S3, so buy it!
If you like to tweak with S3 mod support you can tweak properly everything. For your own or online, everything will be easy or supported. You will benefit from future updates as well. There is already some mod that are very good and way above a baasic vob mod.