BF42.de have posted a Q&A with Battlefield: Vietnam's Lead Designer Armand 'Aj' Marini. The Q&A includes questions about the Eve of Destruction mod, the Battlefield vehicle system and multiplayer issues:
BF42.de: Is BF Vietnam heavily based on the code of BF 1942, or did you more or less reinvent the wheel and wrote a new engine from scratch ? In that context, what does BF Vietnam mean for Mods? Start from scratch, too?
Aj: We didn't completely reinvent the wheel, but the game is completely new to a great extent. Some systems are carryovers, but most of them have been modified. As I mentioned above, the control point system is one that has been changed and there are many more systems that have been changed. The rendering engine is new and everything the player sees in game has been created using new systems and methods.
As far as what this means for Mods, I can say that to a fair extent, Mods will need to be recreated in order to work with the BF Vietnam system.