Wait, so this and the prequel were filmed at the same time?
Let me get this straight. They ended to make this film the sequel, but they decided that it was the better of the two films and chose to make the other film a prequel instead? There's some logic to that the original usually outsells the sequel and presenting the more entertaining one first can be used as a hook to try and bring some of the viewers back. On the other hand, they might have been better served to present it in a Missing in Action: Part I & Part II style. This would allow for greater continuity and a rarity; a sequel which surpasses the original. It might also make it seem like something grander. Instead of getting the story and seeing that which preceded it, you'd get one continuous tale.
Perhaps if the two proved successful, they could have made it a trilogy with Vinh's unseen superior, General Jung, as the main villain. I know that they made a third film, but it featured an entirely new villain to the series and no real continuity. The back-story of him having a child with a Vietnamese woman might even be somewhat contradictory as the child didn't enter into his consciousness until more than a decade later and no mention was made in the other two films.