Canada - I've found that 11 watt incandescent bulbs work the best at preventing this problem.
Pinky and Vmax, you could try running the game in Windowed Mode.
You should have a file called VideoDefault.con. Its location is dependent on your configuration but it should be here in many cases:
C:\Program Files (x86)\EA GAMES\Battlefield Vietnam\Mods\BfVietnam\settings\VideoDefault.con
Make a copy of the VideoDefault.con file as a backup before you edit it.
Find a line that specifies the renderer.FullScreen settings and change it from 1 to 0. If that line does not exist, add it.
Start BFV and it should be in windowed mode now.
renderer.FullScreen 0
Then once the game launches to in a window, you can expand it to full screen by clicking on top window bar and dragging it up to the top of your screen - it should expand the screen to fill your monitor then. Every time a map changes, you will have to re-expand the window.
When you are in windowed mode, you may not be able to move the mouse outside of the game window. Alt-Tab to switch to something else (another application, folder, etc) will free the mouse. Grabbing the top of the window bar is tricky - you won't see the mouse move on it so you have to guess the position and click and drag up when you think you are there. If you move the mouse too far into the window and click, you will have to alt-tab to get out again. Re-read this paragraph after you get window mode working, and it will make more sense.
If you spend more than 15 minutes on this, you may want to consider taking Canada's advice and go buy some 11 watt incandescent bulbs.