I would call VN airlines in Saigon now. My guess is they might just change the date there and then. Probably no need to re-issue the ticket or apply a sticker.

Actually I have never had to pay a fee when changing a return date with Vietnam Airlines - but I suppose it depends on the fare rules and I've never used them to HKG. If they can't help you by phone, then I would just go on your existing ticket without disturbing it although I suppose you could try again at the airport counter. When you arrive in Saigon, go in person to their office (corner of Nguyen Hue and Le Thanh Ton - pretty much in the centre of Saigon) and try again.

You might find www.seatcounter.com useful for checking availability - though its results are not conclusive either way.

Their Saigon's office number is: +84 8 832 0320.