I'm not sure what you mean by a difficulty with "where next"? The fare allows 3 transfers per city, so you can just return to HKG, making it HKG-SGN-HKG. It seems clear that this is allowed (but not on the CX codeshare flight on Air Vietnam, but luckily CX does fly their own metal on this route). So what is the difficulty (presuming these are the real rules and not the rules as shown on the OW web site)?