VS UK site is also saying HK$1635 (£142).
Looking at ITA again it seems to be a country specific YQ thing. If you use GBP on ITA VS YQ comes up as about £160 however if you put HKG as the place of purchase then it's only 998 HK$ (£87)!
If it's a BD miles booking then I guess they charge in GBP hence the higher YQ. It might worth asking to be charged in HK$ as it's a fair difference. VS's own UK site is willing to do it so it must be possible for UK purchased tickets (and indeed correct for the ex HKG flight)