Is there a way to revert Fallout 3 back to a previous version?
Alright, after a lot of head scratching, researching dead forums and reading a lot of documentation, I finally reached the conclusion that the game version was not the issue when it came to VaultMP.
It turns out that the Script Extender that came with VaultMP was for Fallout : New Vegas, and not Fallout 3. I needed to replaced New Vegas Script Extender with Fallout Script Extender. After I did that, the mod worked (sort of) and the error stated in the question disappeared.