My tuppence worth for SGN. On arrival, yes head to the lanes on the left with your boarding pass ready to show them that you've arrived in Business class. Many officials I find struggle with the concept of tier status. So, that's often the biggest variable. Once you are in a line, stay in it and try to avoid chopping and changing as that invariably will get you shouted at by the Boys in Green. Similarly on departure, head to the far right of the immigration desks where you see the official manning the roped off section and just say "Business class" also showing your boarding pass. After immigration keep to the far right for the "fast track" security. None of this is signposted. So, you do have to use some intuition.
Top tip, be ready to go with the flow and whatever you do, do not lose it with the officials. It's Vietnam and they can really make your day a nightmare if they feel in the mood. From my observations, I generally find that in any SGN airport misunderstanding about airline status and fast track, a respectful but helpful attitude in explaining goes so much further than a DYKWIA stomping of the feet.