Because of reasons that I will explain later in this post, I'll begin with the boring stuff: welcome to GOG, enjoy your stay, and don't hesitate to ask if you can't figure something out. We are a helpful bunch for the most part.
If you want an old game with a Vietnam vibe to it, you may want to give
Cannon Fodder a go, depending on what you're looking for - it's not about Vietnam, really, but it begins in a generic jungle area and moves on to other locales later. It's available on GOG, so if you got the hang of how games are installed and run around here, you should be able to get that one working as well.
So, Cannon Fodder.
According to GOG, it was released in late 1993, and I genuinely think that it has aged pretty well. To be honest, I find the juxtapositions in the game really rather disturbing, but that is precisely why I love it. I'm going to open up some of the themes in the game here, so if you want to figure them out yourself, please stop reading now. No hard feelings. Still interested? Right, bear with me.
You can probably see from the screenshots that Cannon Fodder is kind of cute, especially for a war game. Admittedly, Cannon Fodder is hardly unique in this respect (the Nintendo Wars series springs to mind), but the style is as important for the overall effect as is the fun gameplay. Even the title music is called
War Has Never Been So Much Fun (unfortunately missing in the version of the game that GOG got, hence the YouTube link), the really rather grim lyrics ("go for you brother, killing with your gun / leave him lying in his uniform, dying in the sun") sung to a happy-happy-joy-joy pop tune. Everyone in the team has a name, not unlike real soldiers, and even though they have precisely zero personality beyond that, it's enough to make you care about them at least a little bit (losing Jops after ten missions stings)... and yet, they are cannon fodder, cattle to the slaughter, serving only to die. Between missions you are taken to a kind of sit-rep screen which you can find among the GOG screenshots; recruits queuing up for their turn to lay down their lives, with the graves of their fallen comrades littering the scenery where poppies sprout. A slow, reverent tune plays in the background, while a comic relief at the top shows the ratio of own casualties to enemies killed. I think it's a very stirring and morbid scene.
And yet, war is ever so much fun.