I understand. (Sorry, I hadn’t realized that you were just using vitals as a convenient example.)
So, generally within the OpenMRS platform, we can only render obs group members in order if we’re using the original form entry technology to render it. (Because the form does know the order of things.) This is not done in the generic preview (because it doesn’t know about the ordering).
I bet it would probably be possible for us to improve the code in the coreapps module so we get a consistent order for the obs group members, based on the order of the concepts in the concept set that the obs group points to. Would this address your use case?
(If so, create a ticket, and see if someone can work on it.)