Let's be honest in here. The tanks or other vehicles are not included in RS2 because they didn't play a huge role in the portrayal of the war back then and now. It doesn't matter if it is a videogame, an article, a movie or in news. The helicopters and the jungle are a dominant picture.
And when we look at the resources of AMG/TWI, it even surprised me that they realized fully controllable helicopters for RS2. I mean.. if you look at the tanks of RO2 they feel very clunky, rushed and not included well at all compared to the infantry gameplay but they did a good job with the helicopters in RS2 so far in comparison. But the helicopters come with a lot of more work than tanks. They play a huge role in designing maps around them and optimizing the whole game. A larger role than tanks.
They decided not to include any tanks into the game because not many people will await them in a vietnam-videogame. Additionally, they would shift the focus away from the helicopters which many people will stare on a lot.
Not to mention even further map-design hurdles like dense jungle etc.
I understand the decision they've made. The rest of the game like the infantry gameplay and the helicopters wouldn't be that polished with tanks. Btw, many people asked for character customization which is a big thing too.
It's not about historical accuracy at all and it doesn't even matter if they've played a huge role in the actual war or not. It's a videogame after all and a company of limited budget needs to manage the content which is included into the game. And that's the result that we got right now. This doesn't mean we can't change it. RS2 seems to be modding friendly and maybe AMG can include some tanks with the help of the community, even though I wouldn't bet on it.