Hmmm. You're right. The map confirms that Vietnam does have a part right down at the bottom, next to Kampuchea (Cambodia) that does provide a west coast of about 150 miles length. So John Wayne could have stood on the coast of Vietnam and seen the sunset. Looks like it's a rather swampy and rural area, but there are some towns there.

He could also theoretically have found a few other spots here and there along the main eastern coast where there is a little bay or peninsula which provides a western view across water...

So allrighty, then!

But I bet the movie stinks.