If you are on a through flight, i.e. one airplane between your origin and destination, it is one segment. If you are on more than one airplane (with the rare exception of an en route equipment change) it is more than one segment. In your SGN-LAX instance, it is highly unusual to get two boarding passes for two different flights on two different days from one flight coupon -- not impossible but highly unusual. In that case it would count as two segments. (It is also very unusual to have a direct flight between SGN-LAX.)

You can have four segments in Oz but only spend >24 hours in a location after two of them.