Once you have installed Battlefield Vietnam and updated it to Version 1.21, you will need to either change your DNS or your hosts file so that the gamespy points towards QTracker.

This is needed because of course gamespy has been turned off for this game.

https://www.qtracker.com/master-gamespy-hostfile.php outlines the changes you need to make.

Please be aware you don't need to change them all, just the Battlefield Vietnam ones.

simplest one is to make the HOSTS file changes by adding

65.112.87.186  bfvietnam.available.gamespy.com
65.112.87.186  bfvietnam.master.gamespy.com
65.112.87.186  bfvietnam.ms0.gamespy.com

Once you've added those and restarted your machine it should be fine.