All HKG-SIN flights have V subclass sales (unless they changed it within a week as I just did my guarantee this time 5 days!). There are two V subclasses in fact, one with departure restriction and shorter maximum stay of 7-day requirement. For CX659 they offer both V subclasses - one price at $1830 + tax and the other is $3xxx + tax (I forgot the exact figure). You just need to call MPO to get your seat guaranteed!