To update your game, please download the V1.21 patch here: https://drive.google.com/file/d/0B8kuNDnJQWBXQk1JczczOEFHWFk/view?usp=sharing
Once downloaded, (you can download and save it anywhere really) simply run it and follow the prompts. It will patch your game to V1.21. Assuming of course your current game install is V1.2?
Once the patch is done, please download and extract this: http://team-simple.org/download/bfvietnam-v1.21-patched.zip
It is a no-cd crack for V1.21 of the game. Once extracted, you will have a folder titled "bfvietnam-v1.21-patched", go into this folder and copy the contents. The contents being a folder titled "Mods", an executable file titled "Bfvietnam.exe" and the last but not really needed text file titled "Simple". Copy all three and go to your game directory.
Your game directory usually being: C:\Program Files (x86)\EA Games\Battlefield Vietnam if you are running a 64bit Windows.
If running a 32bit version of Windows, you won't have a folder titled "Program Files (x86)" and instead will just have a folder titled "Program Files". If this is the case your game directory would be: C:\Program Files\EA Games\Battlefield Vietnam.
Once in your game directory, paste the copied items and replace them if it asks you to. Once done with this, delete any shortcut you may have created for the game and then create a new one from the Bfvietnam.exe launcher that is now in your game directory.
I trust this helps.
If you need any other assistance, please feel free to get in touch!