Didn't feel like they were in Vietnam
I saw this in the cinema in London back in 1987, and when they went to Vietnam I remember feeling that there was something very familiar about it.
I now realise that it was largely filmed in East London, and the weird vibe I got was from the weather and the sky. It was exactly the same as the conditions I was used to seeing every day of my life. Albeit without the gunfire and tanks etc.
Also, Vietnam is a very green, tropical country. The 'Vietnam' in the movie looked like a piece of derelict London waste ground with a few palm trees placed here and there (which it was).
I guess Kubrick had his reasons.