According to this story on Gamespys PlanetBattlefield, Digital Illusions, the developers of Battlefield 1942 registered Battlefield Vietnam on February 12th, 2003.
I would also like to spotlight a great looking BF1942 mod currently under development called Eve of Destruction, The Indochina Vietnam Conflict (Whew!). I have only seen screenshots posted on their website, but I cant wait to get my hands on the game. They have some incredible custom terrain modeled, as well as great looking soldiers. You need to keep your eye out for this one, even if Dice decides to make their own Vietnam Battlefield.
Oh yea, check out their website.